|The hard drive probably must be jumpered master when it's in the external enclosure.|
If the drive is by itself on a data cable, jumper the drive Master, or jumper it Cable Select and have it on the END connector of a 3 connector data cable. Some bioses won't recognize the drive at all if you don't do that.
If there is another hard drive on the same data cable, some hard drives - e.g. many Western Digital models - have two ways they can be jumpered as Master - Master, single, or similar, for when the drive is on the cable by itself, or Master, with Slave or similar, for when the drive is on the data cable with another drive set to Slave.
That MUST be correct for the situation.
Don't mix Cable Select and Master / Slave jumpering for two drives on the same data cable. Some combos work, some DO NOT allow both drives to be recognized.
That drive does not have to be connected to an 80 wire data cable if no drive connected to the same data cable requires it be connected to an 80 wire one, but 80 wire data cables must have the proper end connector connected to the mboard IDE header - usually that's blue, but in any case it's the one farther from the middle connector on a 3 connector data cable.