After receiving advice from you guys, I think I will opt to go with: Disk 1 - OS and my Files Disk 2 - Time Machine to back up Disk 1 Disk 3 - smaller, slower disk for non important files such as trailer downloads etc I have a question about time machine though.... how does it work? If I set time machine to back up my disk one, and say I install a program that messes things up a bit (and simply deleting it from my apps doesn't fix the problem) can I 'just' revert the OS files? Or will it revert all of my own saved files, such as photos too? Or is it more like a snapshot, where I can pop in and take the files I want from a particular time and copy them to my current state of HD? Which is great for copies of a file you're working on but how would that help when the os is playing up? And finally, if my main HD dies, can I simply copy my personal (not OS) files onto the new disk? (I guess so but just checking) so for eg, if I wipe disk 1 and install snow leopard on it when it comes out, I could simply fetch my personal files from disk two? Thanks in advance!