USB 2.0 HD's Dissappear

I'm running XP pro SP-1 on a Gateway PC. I have 3 HD's NTFS connected to USB 2.0 ports, letters "O", "P" with a separate extended partition "Q", & "R" with a separate extended partition "S". When I power on HD "O" it runs normal & I can access & save documents, & safely shut it down. HD's "P", "Q" & "R", "S" are different. When I power either on, either will run normal & I can access documents, but at times (sometimes right away, sometimes an extended time, no warning) they just dissapear in Explorer, Disk Manager (rescan disks doesn't restore), & Safely Power Down HD Tray Icon, preventing me from saving documents.

I power off the HD, re-boot & XP gives me a recovery from a fatal error message, & isn't always a normal boot, so sometimes I end up booting 2 or 3 times before XP behaves normal.

I swapped the USB ports with HD "O" that works, also changed drive letter assigments, but either doesn't solve the problem. Any ideas?

Thanks...Don