As far as installing debian on it, it can be done. I'd highly recommend setting up the serial first.
Then follow the steps from this post
Debian on LS Pro Duo: viewtopic.php?f=50&t=23971
but use the patchset for the Quad instead of the duo: viewtopic.php?f=66&t=26035
This is a pretty quiet site so doesn't matter too much, but may be best to ask/add your input into the LS-QL subforum.
As far as CrashPlan, I have never used it, but unless you have a particular love for it, I'd expect more problems - not to say its impossible, but certainly not supported. And with the importance of secure, verifiable backups, why risk it? Crashplan states its linux system requirements to be 1GHZ+ x86-64 CPU, 1GB+ memory, 50MB+ free drive space, Oracle (Sun) Java version 1.6+, so basically looks pretty bad for a 400Mhz arm platform with 128mb memory. Also debian now has officially dropped the Oracle Sun java in favor of better licensed alternatives, which may also throw a wrench at it. Looks like this person is working on it however http://pcloadletter.co.uk/2012/01/30/cr ... o-package/
So I would suggest to go with well tested open source backup software, that can be installed through the package manager (once you have debian on it). If you are going for off-site backups, maybe you can hook it up to amazon's glacier storage http://aws.amazon.com/glacier/
, simple storage (S3), dropbox, or the like. It can't be much more work than getting CrashPlan to work, likely to be more cost effective, and certainly more fun and reusable.
something like http://blog.eberly.org/2008/10/27/how-i ... ing-rsync/
Write back with your results or if you need any help. I check on here every week or three.