[dnanian]

While the image itself can be compressed at approximately 2:1, you need space to hold both images when creating it. So, if your 160GB RAID has 100GB of data, you'd probably need 150GB of space, maybe a bit more, during the backup.

Of course, you could create a sparseimage and backup into that, then do the conversion "by hand" onto an alternate volume... depends on how tolerant you are of these kinds of things.

But, for ease of use & simplicity, I'd have a 160GB external FW drive and just backup to it, then Smart Update subsequent times to ensure everything's up to date.