I'm new to mac and just got a new iMac. It's great and I want to do my 1st Time Machine backup. It tells me that I have 23gb to back up. I want to use a USB (I think) for the backups. Is it okay to use 2 16gb or do I need to get a 32gb? I don't need to keep backups forever. I just want current snapshots that are up to date.