A word of caution- do make sure to back up to more than one external. How you are suggesting to work is how I was working, and mirroring my photo library to a second external drive. It worked well...until the main external failed. The mirrored drive reflected the lost files- which was much of my library. As a result I lost my entire on-site photo library. I was sick. Thankfully I have a subscription to BackBlaze and as soon as I realized my library was corrupted and I could pull nothing off of it, I ordered a hard drive from them that contained it- fortunately for me the failure had happened pretty quickly and the 4 weeks of stored mirroring of my drive that BackBlaze provided containted my whole library. It was worth every penny of the $200 the drive and restore cost.

My point? Place a high importatance on redundancy. I was told that external drives fail more often/are less stable than internals. I now have the drive Back blaze sent me (which I will update monthly and store off-site), a 2nd new internal drive in my CPU which is now my main photo drive (Western Digital, Black Caviar), one external that will mirror my internal (I will be getting another external to also mirror my intenal with pics), AND I have BackBLaze.

So far as catalogs- you can set it to store a back-up copy on another drive, or even to a folder that you link to DropBox.

Joe,

Take a look at CrashPlan. They have unlimited plans. I recently switched to them from Carbonite (also unlimited), except Cabronite throttles your upload speeds to ridiculous (almost useless) rates when you get close to 200gb (less than 1gb/day upload). I have 178gb backed up on CrashPlan (all my photos, and catalog file) as well as local backups for faster retrieval if ever needed. I was able to move all 160gb of storage (my total when I opened a plan with them) to CrashPlan in a little over 2 weeks.