Currently I have a small photo catalogue that still fits on one computer - slightly under 400 G. I would like to provide for
1. Time machine backup on-site
2. Off-site clone / backup drive(s)
3. (Soon) migrating a portion of the photo storage / LR catalogue to on-site external hard drive, ideally with fast access
My backup system has been Time Machine (excluding Lightroom) plus daily LR photo/catalogue backup onto the same backup disc (2 TB FireWire 800) and LR photo/catalogue onto a second 2 TB FireWire 800 disk theoretically to be stored off-site. No clone disc.
FW 800 disks available: the 2 2 TB disks mentioned above. These are 4 or 5 years old. I don't keep them spinning all the time. I turn them on and hook them up when I have photo ingestion and editing to do, which might be 2 days a week.
USB 2.0 - only discs available, all spinning rust: 1 1 TB, 1 500 G. Possible use for clone backup disc?
The computer these served had FW800 and USB 2.0.
Now I have a Thunderbolt 2 - capable computer, a 2015 MBP.
Would it make sense to get a Thunderbolt 2 large-ish (2 TB) external drive for photos not resident on the laptop, for use during editing, and keep the backup system as described above?
Let me explain what I do and why: I have a single MBP for all my computing needs (aside from an iPad Pro and iPhone 7+) and I have one external USB drive (now USB-C) where all my images are stored. The Lightroom catalog is on the internal 1TB SSD drive of the MBP (160GB for catalog and previews for 89000 photos).
The backup strategy is: The MBP is always backed up in my office on two external USB drives using Timemachine (TM). The external drive for photos is backed up on two drives, one sits in my office and one is a portable drive that I bring on my trips. I synchronise (not backup) the folders on the external drive to the two other drives using ChronoSync. Before I leave on a trip I make a final TM backup on the two TM drives and sync the image drives. On a trip I bring two portable drives for TM backup during my trip and I store all photos I shoot on the trip on the internal drive on the MBP. On the trip I try to keep the MBP and backup drives separate so I would never loose all in case of fire, theft etc. If I have at least one TM backup I can take back (if I lost the MBP) I can restore the entire MBP content to a new machine and it will be in the exact same state as when the last TM backup was made. If I loose everything on the trip I can buy a new MBP and restore all from one of the TM backups in my office. Why two in the office? If one of them fails then I like to have a another to restore from.
In addition I have an online backup of the MBP and the external USB-C image drive on Backblaze.com. I currently have 3.2 TB backed up. It costs $5/month for unlimited backup and I have this service since 2010. When I return from a trip with e.g. 200GB of photos it takes a couple of days to upload with the 100Mbps symmetric internet fiber connection I have. I hope I never need to restore from Backblaze but in case they offer to send all backup files on a USB drive. You can also download from the site, of course.
When I need larger HD's as I grow out of the current HD's I don't keep them except if they make sense to use as TM backup drives. I never exclude anything for TM backups because a TM backup is the basis for restoring on a new computer or alternatively connecting the new computer with the old one in target mode and the restore directly from the older MBP. The benefit of this is that the new one will look exactly like the old in terms of settings, apps and data.
Unless you have the need to use the catalog/photos to multiple computers, I would not see the reason to put the Lightroom catalog on an external drive which means you always would have to have the external drive connected to the MBP when editing photos. Especially when I travel I don't like to have an external drive connected all the time. One option that would work in this case is the new Samsung SSD T1 or T3 drives which are very small and light and don't get easily detached from the MBP. I had this option in mind when I did not go for the rather expensive 2TB SSD option for the internal drive on the MBP 2016. I don't need it now, but perhaps in the future and there is a 1TB and 2TB usb-c model and the speed is up to 450MBps.
I those a TB2 external drive before with the 2014 and 2015 MBP's I had. It's the LaCie 2 drive RAID. I regretted this since it is quite noisy. I got the new LaCie 5TB USB-C drive to use with the MBP 2016 and it is very quiet which I like.
I'm sure your needs are different than mine and solutions depends on requirements, so I'm not trying to say what I do is the right solution, except it is the right solution for my needs. So take from this description what makes sense
Remember I run a business so I can never afford to loose all I have.