If you're going to put important Mac files onto it, I suggest that you initialize it "for the Mac" using HFS+.
Things just work out better that way.
For cross-platform (Mac to PC) compatibility, use a DIFFERENT drive of some sort, such as a higher-capacity USB flash drive.
One last thought:
If the files you put on it are valuable, then you MUST also keep a copy of those files on ANOTHER drive.
One copy is not enough...