I just bought my first Mac (2.93 GHz iMac) and I bought a brand new MyBook 1TB to use as my Time Machine. I upgraded the firmware to try to avoid any problems with the Firewire that I've seen reported. From there, I forgot about the stuff I had read about changing it to GUID from MBR. I just plugged it in and let Time Machine delete it and start doing it's thing. Am I at any kind of disadvantage? I have a lot of stuff on it, but it's only 1 day of history so not a big deal to start over. I want to do it now if it's the right thing to do. Thanks for any information.