I'm not sure if there's a tutorial on mac/xbox but - i've been messing around with several different programs lately...and they all seem to do the same thing: the ability to read an already existing xiso (disk image you've created of an xbox game). this would be helpful if i wanted to unpack an xiso and dump all the contents on my xbox harddrive but, i'm in the same boat as you - my harddrive lacks the space. but, to be honest - prefer backing up to dvd anyways.
Maybe it's something to do with the xbox but, why someone hasn't made a program that simply connects to your xbox and backs up your d:/ into an xiso is beyond me (i think Xbox Backup Toolkit can do this but, i can't get it to work correctly???). With that said I think I may have found a decent way. FTP.
First off; I have no modchip / I don't know all the tech terms but, my friend and I put the MA save on a memcard and I loaded it in my xbox; then from there i ftp'd into my xbox and took over the c drive with evox and phoenix...
after i took over the c drive i'm not able to read the d drive in my ftp (before with just the audio/cd hack i couldn't - d would replicate e? or some other drive).
i tried yesterday to drag all over d over to my mac and it was working; but, my wifi connection timed out while i was gone. I'll keep working on it over the weekend and post results soon.
hope this helps!