Disappearing Drive

Hello All,
I've had this problem for awhile, but here goes.
I have an HP computer its a few years old. I have two
optical drives on it one internal and one USB external.
For some reason the external drive sometimes disappears
from the computer. I can't find it anywhere then a few days
later it pops back up. I have tried replacing the cables and
checked to power and still the problem continues. I also tried
reinstalled the system and sometimes it works.
If anyone has any help it would be appreciated.

RalphyG.