Mac partitioning problem

[dranorter]

I am following the instructions from Wiki.debian.org/MacBook to set up my MacBook dual booting with Debian.

The first step is giving me trouble. I have a 55.57 G partition, with 23.9G free, and I want to put Debian on a 20G partition, so:

Sylph:~ dranorter$ sudo diskutil resizevolume disk0s2 35.57G
Password:
Started resizing on disk disk0s2 Sylph HD
Verifying
Resizing Volume

Resizing encountered error No space left on device (2 on disk disk0s2 Sylph HD
Sylph:~ dranorter$

What is the problem here? Why wouldn't there be space on the disk?

[dranorter]

Okay, I gave in and actually downloaded Boot Camp. It returned a more detailed error:

Your disk cannot be partitioned because some files cannot be moved.

Back up the disk and use Disk Utility to format the disk as a single Mac OS Extended (Journaled) volume. Restore your information to the disk and try using Boot Camp Assistant again.

I've had trouble finding a good backup utility, though. SuperDuper freezes an hour into backup. Can someone recommend a utility which will efficiently save ALL my files, settings, and applications and restore them once I've reinstalled OSX?