While in SystemRescueCD and using partimage to backup an ext3 filesystem, I kept running into the error:

Error - Cannot create temp file [/dev/sda3/pi878f5c20.tmp]. Please check there is space enough and you have access rights.

The problem is that the hard drive/partition needs to be mounted. Do the following when booted into SystemRescueCD

mkdir /mnt/sda3 (you can call sda3 anything you want. It's just a folder that you will mount the actual partition you need to store the backup onto)

mount /dev/sda3 /mnt/sda3 ( this will take the partition /dev/sda3 and link it to /mnt/sda3)

After doing this, navigate to /mnt/sda3 and see your folders.

There is a good how-to for Partimage here. It also describes how to restore your image to the partition.