If you are really, really want to do that you need to read these first:
- The up side: you can read and write to it from Mac and PC.
- The down side: the files size is limited to less than 4GB each. Meaning if you try to copy a file size bigger than 4GB it will give you an error or not work. You can not use Time machine with the drive. If you allow Time Machine to configure the drive, all of your data will not be read or write from PC.
If you have decided to go for it,
You must back up all of the data to either your PC or MAC internal hard disk drive.
Formatting the drive will erase all data.
After you have done backing up, connect the Passport drive to your Mac computer.
Bring up the Disk Utility,
Select the Passport drive,
Select Partition, Under Volume Scheme choose 1 partition,
Under Options, choose Master Boot Record and hit OK,
Under Format, choose MS-DOS (FAT), then hit Apply to format the drive.
After that the Passport drive is formatted and it is able to read and write from Mac and PC.
If you right click on the drive (in Mac OS X) then click ‘Get info’; the "Sharing & Permissions’ caption states
It will show read and write.
Now you can take it to either Mac or PC to start writing to it.