Quick story:
I bought a new 2TB My Passport HDD for my video files/etc.
Ordered an iMac on Sunday and the Passport HDD had no use anymore because I work with a Windows laptop to record footage and the Mac will require to reformat the drive in order to use it.
So I returned the HDD + soft case I bought back to Amazon and got the My Book Live from Best Buy at almost the same price + free shipping. So not bad.
I heard a lot of negative reviews on this thing but I had no other choice because there isn't an external HDD that doesn't require formatting when switching from OS back and forth.
So far I hooked it up and everything is going smoothly with this thing. I'm loving it. I now have a network where I can stream video/music/pictures to my iPad, PS3, etc. and share files via Internet/Wi-Fi between the computers.
2TB My Book Live btw.