I just have a 15inch rMBP so it’s obviously not similar to your situation.  But I have a 6TB (two mirrored 3TB) LaCie external HD on which there is one main folder for all of my Lr photos.  This includes all of my former Aperture photos (which was a combined old iPhotos library and Aperture library).  Since the beginning of 2015, all of my new photos are purely Lr. So they are all in one folder on my external HD.  When I travel with my rMBP and I’m taking pictures, say on vacation, I’ll create a separate Lr Library and do all of my post processing in it with a smaller 1TB external HD as backup and then when I get home merge the travel library with my main library and move the photos into my master folder.  Julieanne Kost has a video on this.  The interesting thing about the video is that it shows how you can integrate several different computers with just one master file at about the 16th minute mark of the video. If you attach the external HD to your iMac, I do not believe, that Time Machine will back up files on the EHD.  You’d have to have an off site cloud based back up system (I use Backblaze) to make sure you can access a source should your iMac crash, get stolen or damaged.  I use another external HD to physically copy my master Lr folder to and keep that off site as well. Joseph ingrained that paranoia of multiple back ups into my brain a long time ago. :-)   A network system might be the easiest with regards to using several pieces of hardware together, but I know very little about networks, so maybe someone who does will chime in. Hope this helps.

Florian

Pondball, My external HD is really two 3TB HDs mirrored so that if one crashes the other will be available.  So I really only have 3TB of storage which for my usage is just fine for now.  It does run at 7200 and is attached to my rMBP which sits on my desk 95+% of the time.  Only when I go out of town for several days or more mainly on travel/vacation/business, do I take the rMBP with me and I already have a separate lrcat for travel set up on my rMBP HD and a 1TB external HD for travel which I already had mentioned.  My rMBP is were I house my main lrcat file, as well.  So the photos are on the EHD and Lr is on my rMBP. I’m anxiously waiting for Joseph’s Lr for Aperture User training video.  I’ve set something up which is close to what I had in my Aperture Library with a slightly different variation for Collections.  The process probably could be a little smoother so I can’t wait to see how Joseph approaches Lr, Glad I was able to provide some help.

Florian

Just some thoughts. I’d do organization in Finder, not LR. While I’ve found LR stable and capable, it’s not as convenient as positioning and moving Finder windows on your desktop. If you’re as dispersed as you indicate, you’ll be spending a lot of time in LR just locating images.
If your images are already rated and keyworded, you want to either create side cars or, use Aperture to suck them all up in a referenced library. Then import that Aperture library into LR. I moved from an Aperture referenced library to LR. It went well. Start with some test libraries and import them to see what LR is doing. Look in the Finder, understand what you’re looking at. I did not care to have LR create a separate, apart from my organization, folder for all edited Aperture images. I exported tif’s of all Aperture rated images higher than 3 or 4, forget which. Exported to the same folders the masters were in. Then set LR to not create the Aperture Previews (or whatever they call them) upon import. If you go with the Adobe approach you may very well end up with a very disorganized mess of edited images you will never put back in their appropriate folders. I edit a lot but I hand out stars very frugally. For me, my approach worked much better. I can give you my hardware layout. It’s not the right answer as there is no right answer. But it works for me and I have no complaints. The Mac is dedicated to photography and streaming media throughout the house. I use an iPad for just about everything these days. rMBP + 25” Dell + 2 OWC dual Thunderbolt enclosures. 1 TB internal, all LR catalogued masters located here. Dualie #1, 1 TB ssd for all other data and non-catalogued images, 2 TB partitioned spinner to backup each of the former. Dualie #2 is nothing but media including some duplicates of images in the former drives, 2 4 TB drives, nothing backed up. 2 usb3 2.5” bus driven drives, clones of the backup in Dualie #1, stored in a safe deposit box. Convenient. I’m not ready to embrace cloud storage. More from a convenience pov than security. If I have a calamity that takes down primary and backup drives, 1), recovery probably won’t be my number 1 priority and 2), it’s a simple process to pick up the drives and both use them and clone them back when the primaries are replaced.

In the above scenario, odds are there won’t be any internet to do me any good with cloud storage. I live in a hurricane zone. House is on the water.

Sorry about the lack of paragraphs but that’s what happened when I edited a few words.

We have 2 homes and having desktops in both places was a pain, maintenance and sync. So, yes, I now have only the rMBP and displays at both ends. Chucked a half dozen enclosures and run OWC enclosures that hold 2 3.5” or 2.5” drives each (dualies). Been about a year now and delighted with the setup. Both from a travel pov and everything in one place. When we’re at home #2, 2 2TB 2.5” bare drives go with me and I have an empty usb3 dualie they are mounted in. We tend to travel a lot when we’re there so this year I haven’t even bothered using the external display. It’s not pleasant but I’m using the 13” display on the rMBP.

I am using standalone LR 6.1.

With LR’s smart previews, you can pretty much decide where you want your primary image storage to be. Say it’s on your iMac. Then for those files you want to edit on the MBP you create smart previews. You would copy the LR catalog (and perhaps a Smart Previews folder – don’t know as I don’t use them) and work on those on the MBP. I believe it’s a good feature but you would have to brush up on it as I don’t have any experience with it. There are all sorts of ways to deal with multiple machines. Just need to find those with experience and their thoughts. But yes, the catalog would be on both machines. Either the same catalog or parts of it.

I usually run pretty new AE’s. I have never found file access even remotely acceptable to me. Wake times are slow, access times are slow. Have not tried it in years but I’d test before you get very far into this approach. It may not be cutting edge but attached storage is cheap, fast and a lot more reliable. 7,200 rpm used to be important. With today’s large drives, platter density can get you more speed than rpm’s. My 5,400 rpm 2TB Spinpoints on usb3 can outrun my 7,200 rpm 2TB Hitachi on Thunderbolt. Unless you’re using old drives – which should be replaced anyway – I would not worry about rpm’s. Drives that are 75% or less full and fast I/O, usb3 or Thunderbolt, have changed the old metrics.

You’re running into the same issues I dealt with for years. I looked at how I and other people handled things and came down to one machine is simply a better solution for me. I’m not a pro and don’t need a 16 core Mac Pro with multiple externals, etc to get the job done. You might want to give some thought to just using your MBP. I use an Arc stand and it’s a snap unplugging one USB cable, one Thunderbolt cable and the power adapter. I have a 30,000 image library, all originals are on the internal, I limit 2880 previews to 20gb and am using 504 GB. Music, when we travel is a downsized iTunes library on a 128GB Jet drive (short sad card).

If you’re not already vested in a lot of Keywording and you have not used Aperture all that much, you’re not gaining much by importing an Aperture library into LR. If you already have tif’s or jpeg’s for your Aperture edits, you may want to: 1), get your organization done in Finder. Import into Lightroom. The intermediate step into Aperture is not saving you much time. If you are using Managed libraries in Aperture, iPhoto, Photo you would want to first switch them to referenced. Give timing some thought before you start. Once you import part or all into LR, you want to stop using that/those libraries.

If it’s of any assistance I use a modified approach to organization. 80% of my “events” are in date order. Then I have a series of folders for subject specific categories. (family and friends, interests, etc). I also don’t catalog all photos. For those that are very subject specific and access very infrequently, I keep them in folders on the external ssd. I access them with a variety of browsers (currently Graphic Converter). I like to keep as little as necessary in Lightroom.

You know your data requirements better than I do. When we travel, my 1TB rMBP and my wife’s 256 GB Air travel with us. DropBox is used extensively for anything shared, iCloud for the usual iOS syncing for mail, calendars and contacts.

I tote along a very small OWC Express enclosure with a 2.5” 2TB Samsung Spinpoint. It’s partitioned for the rMBP (750 GB) and Air (200 GB) backup clones (SuperDuper) and a 1.1 partition that contains my external SSD and some extra space. It’s a very small unit and more than worth the extra space. I’ve been using SpinPoints and the Express enclosures for years and they have been 100% reliable.

We travel an easy 2 to 3 months every year. One home is in the USA, the other in Europe. The above works for me.

I keep my photos on a network drive.  The NAS is reliable and redundant, and I move the files off the cards onto the “images” share for safe keeping and permanent storage.

I use gigabit LAN, which is the norm these days—just make sure your switches are gigabit and your cables are good enough quality.

It is not too slow.  Initial cataloging is slower than I would want, but working in it with develop settings, metadata, etc. is no problem. 

I back up the file server, or the “images” share, onto a bare HDD.  I don’t need a fancy name-brand enclosure; just drop it into a “dock” and pull it out when I’m done.  The NAS is redundant against drive failure and uses ZFS snapshots to protect against human error: image files don’t change once stored, so a snapshot doesn’t consume any storage and I set it to hourly snapshots when I was worried about messing something up in a Premere project when my desktop seemed to be having RAM problems.  Since it adds trivially to the storage I’ve left it that way ever since, and I’m “safe” if I wait an hour after dumping the cards, against accidentally deleting the file when curating them.

you ask, “Where can I host my main photo folder on my network that will make it accessible from all my machines.”  If I understand that correctly, you want a file server.  Either a dedicated file server machine on the network (like I have), or some other machine that’s always on and doesn’t move, that can also be the file server even if you’re using it for something else.

There are enclosures sold for that purpose now: you plug it into a network switch rather than into any single machine.  They are overpriced and the RAID features are not as robust as you would hope.  An old machine can be turned over to the task, just leaving it on, or downloading special (free) NAS operating systems for it. 

If you have an old machine, left over after an upgrade, just park that in a closet.  Attach the drives to that.  Or, scrounge around for one from family and friends.  A too-old slow laptop with too-little RAM for the lastest OS upgrade and apps can run a lean mean Linux image just great and find new life with a new purpose. 

The tricky part, really, is the RAID stuff.  You want to set up a drive-level redundant system, or a system to automatically replicate the files onto other drives.  An old Mac has Time Machine!  So problem solved there, with a main and a backup drive.  If you have a place to plug in more drives… no “internal” connections for SATA drives there.  A hand-me-down PC motherboard with (say) 6 on-board SATA connectors and the firmware booting from a USB flash drive would give you lots of capability.

To recap,

• network-based disk is not too slow
• arrays or otherwise getting automatic backups for redundancy does not need to be expensive (beyond the cost of the bare drives: $30/Terabyte as of my purchase last week)
• any old desktop PC can be salvaged to make an always-on server that can hold a bunch of drives at full SATA speed, with free software.  A five-year-old PC can probably be found for free, sans display. 

More thoughts on salvage:

• Old laptops may only have one “eSATA” connector and predate USB3, and would need a cheap enclosure for the drive to give it power, so don’t count on an old laptop to be a good fit.  But you might find something that works.
• And old desktop PC can have a new SATA-6Gbps (usually marketed as SATA III or 3 against the wishes of the SATA consortium) added for cheap, or a SATA-2 card added for dirt cheap.  SATA-2 is faster than any HDD.  Google “newegg sata controller” and I see 4 Port SATA II for $15.  That’s what you need to upgrade a super old PC that doesn’t have enough ports.  I wouldn’t go with one that was so old as to not have SATA II built-in at all, and any motherboard would at least have 2 of those, which is enough for the primary and a backup drive.  Remember the firmware image boots from a flash drive.

Note on sourcing drives:

• Buy “bare drive” 3.5″ SATA HDD.  Usually some with less than top-of-the-line performance can be found on sale.  You don’t need a fancy name-brand enclosure, just the drive.  It connects to the motherboard and a few of them ought to fit in any old desktop case.
• Get a SOHO RAID drive (e.g. WD “Red”) for the primary, and the sale drive for the backup.
• Any current 3.5″ HDD, even the cheaper ones, should have >100MB/s sustained throughput, so that’s all you need to fully use up the gigabit network connection.

sorry if that’s long, but was following your example!