the CrashPlan client does not offer this basic level of functionality. I had a similar problem and I can confirm that it is not possible, according to tech support at CrashPlan. They are either unable or unwilling to support it, instead referring paying customers to the forums for help. Here is the email I sent explaining a very similar situation, along with their response:
Crashplan, May 22 09:49 am (CDT):
Ubuntu Linux 12.04 64-bit, fresh install.
Root filesystem '/' is on a small SSD
Home filesystem '/home' is on a large HDD
Backup drive is on a third HDD.
First drive automatically mounts at BOOT, next two drives automatically mount at LOGIN.
CrashPlan starts at boot, before I login, so it creates a directory in the small Root drive, which quickly fills up. I need to clear all those files out, and then set CrashPlan to wait for the HDDs to mount before it writes any files to backup.
Jason S., May 23 11:17 am (CDT):
Unfortunately, CrashPlan is specifically designed to launch at
startup, at the runlevel that it is assigned. While it may be possible
to disable that functionality and cause it to start manually, that's
not something I would be able to assist with as it is not a supported
Here is a link to the forums:
Our user base has come up with means to assist each other with several
unsupported configurations. I would suggest that you inquire further
on the forums.
Let me know if there is anything else I can help you with.
I was quite surprised to see CrashPlan creating a new directory at root, then blindly filling the disk until it created a bigger problem. It's my opinion that the software is poorly written and is not suitable for use until this is corrected. Déjà Dup will check that the destination drive is connected and has space, which is the correct approach. CrashPlan gives no error messages and creates new problems.
I recommend using Déjà Dup combined with Ubuntu One instead. It actually works, and does not create any new problems. It can cost more if you have lots of data to back up, but you get what you pay for.