Default Backups?

Yes, I'm a noob with a noob question. I just rooted my wife's Fassy last week to finally make it work like it should (thanks to sfobrien). I'm trying to create a backup in CWM with no luck. Here's the message I'm getting:

CwM Voodoo Lagfix Recovery v3.0.2.8x.ti
SD card space free: 13103MB
Backing up boot image.....
Error while backing up boot image!

Could someone tell me what I'm doing wrong?
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