Well you can format the drive FAT32 and it will work on both but the biggest file it will accept is 4 gig. You can formatt it NTFS (for Windows) then download and install on your Mac NTFS-3G http://mac-free.com/download/NTFS-3G.html and it will allow the Mac to read and write to the Windows formmatted drive. NTFS-3G is free the similar software to make Windows use a Mac format drive is pay.
Joe
