formatting a toshiba external hard drive for mac and pc I tried to format my Toshiba external hard drive to accept Asus pc and MacBook air files. I removed everything from the hard drive, put it on my pc.  I used the format MS DOS (FAT} on the mac and erased the hard drive. All was well till I plugged it into the Asus and now I can not open the hard drive to put the files back on it.  It shows it is there but does not open.  Just want to add that I used time machine on the mac and I believe that is causing the problem. I would really just like my files from the pc back on the hard drive.  What should I do? Erase everything and start over? Is it possible to make the mac and pc compatible on the hard drive. I am not computer savvy and would appreciate any help! Thank you.
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.