Microsoft Backup doesn"t see external drive while Explorer does se

I am new to discussion groups. If my question is not posted to the correct
group please let me know.

I would like to use Microsoft Backup but it doesn't see the USB connected
external drive (F:). Windows explorer does see it. Also I copied successfully
files and folders from C: to the external F: drive.

My environment is Windows XP Service Pack 2.
Any suggestions will be appreciated.
Thanks,
xan