formatting a toshiba external hard drive for mac and pc
If you format is as "FAT" then you'll have lots of issues with the size of files you can write to it and other filesystem limitations. Best bet is to format it on the PC as Fat32 or even better exFAT on the Mac. 

Both of those will give you read/write capability on both the Mac and PC. Just remember tho when you first plug it into the Mac wait for Time Machine to pop up the question about using the drive to do backups. Say no and it won't bother you again.