I looked at this enclosure on Amazon (
http://www.amazon.com/Taurus-Mini-Super-enclosure-only/dp/B00GRHY7UE).
Seems like a pretty good price for a FW800/eSATA/USB (2 or 3?) enclosure for 2.5" drives, but it appears to be a "bus-powered" device (i.e., there's no power supply/adapter). If that's true, it
may require too much power to work when plugged directly into the Time Capsule, as I believe Apple specifies for the TC and Airport Extreme that the USB port can be used with a self-powered USB drive. You could plug a
powered USB hub into the TC, and then this drive into the hub -- that should work. I would look into a 3.5" drive external myself, as it will be powered and I don't see any advantage to using 2.5" drives for this use (they are usually more expensive per GB).
One thing I noticed in the Amazon listing is that the text says it has USB 3.0, but the image of the back shows USB 2.0. It would make absolutely
no difference in performance when used on a TC, but it sure would if in the future you connected to a computer w/USB 3 port. Just know what you're getting.
I assume you're considering it for the RAID 1 (mirroring) capability. Personally I'm not sure that's worth it, as it seems I've read enough horror stories about RAID controllers causing errors or failing that I'm not convinced that even with the HDD redundancy you're really getting an overall more reliable solution. But that's up to you, and I don't claim to be an expert on RAID devices, by any means.
I guess maybe you're going to use the TC's internal drive for TM backups, and this (or some) USB for external storage on your network? Then it makes some sense to use RAID 1, I guess, as Time Machine (the software) cannot back up any drives plugged into the TC (nor any network-connected drives, I believe). But you're totally relying on the RAID 1 feature to back up any data you store on this "network" drive. So you're only protected against one of the HDDs itself failing. Hmmm.
Also, I totally agree with "
cruisin" above -- it will be pretty slow to access your "external storage" over the network. Probably especially slow if the TC happens to be in the midst of a backup to its own internal drive, because I believe the TC's processor is the bottleneck to disk throughput (when connected by 1 GBethernet). My TC is an old 1st-gen one so maybe this has changed, but the best thoughput I get (again, wired, not WiFi), is around 10 G-bytes per second. That's less than 1/10th of a HDD throughput, isn't it? So whether "it works" for you depends upon what you are using the storage for, I guess.