[peterb]

Macs use a different file system (HFS) for their hard drives, however they can read/write to FAT file systems, and I think NTFS (Windows).

If the drive is going to be used exclusively with the Mac, then reformat it to HFS on the Mac. If you are going to share it with a Windows based machine, format it to FAT32.

Some USB powered drives can be unreliable if the USB port cannot supply enough power. The spec allows for 500mA, but some provide less/some drives take a bit more. You can sometimes overcome that with a double ended USB cable - it has two plugs at one end to plug into 2 USB ports (one just supplies extra power) and a single plug at the other for the drive.