Try your experiments with the SCSI terminator not on the enclosure. It is possible that the drive in the enclosure has a jumper set to provide termination, so with the added external terminator, you could be double-terminating the external end of your SCSI chain.

Alternatively, check the jumper settings of the drive in the enclosure.

Also, if these are Seagate drives, make sure that "Termination Power to Bus" is disabled. The host (the Macintosh) provides SCSI Termination Power, and providing it from additional devices can cause arguments amongst the voltage regulators involved leading to problems.

Even if they aren't Seagate drives, you should do the above, but it's only with Seagate drives that I've ever found it to be a real problem.