Try it on another PC if possible.
If it's still not detected, it may be the adapter board in the Toshiba casing that's faulty, as that's a common problem with external drives of all makes. The fix is to extract the drive from the casing (often a mission in itself!) remove the adapter board, then buy a third-party external enclosure for the drive.
Normally I would recommend you test the drive with diagnostic software before cracking open the Toshiba casing, but if it cannot be detected by the system, the the testing software won't detect it either.