Double termination is also a potential pain if you at any time wish to add another SCSI device, such as a CD-ROM drive, to the chain on the remote side of the external HDD. An external terminator is easily removed for that purpose, but not so an internal terminator. The internal terminator could take the form of a jumper marked TE (termination enable) on a header block, especially on the Seagate drives that trag mentions. Alternatively, and depending on the age of the drive, termination could be passive, via two (or three) removeable in-line resistor packs on the drive's logic board.
SCSI is as much art as science, in which there are certain truths (don't put LVD devices on an HVD chain), frequent truths (all attached devices should be powered-on at startup), sometime truths (all SCSI cables are equal, but ultra-skinny cables are very unlikely to be so), and barely truths (Apple always implemented SCSI in a consistent manner).
You didn't say which model of Mac was concerned in this case, but the odds are that it uses the same bus (ie, with a single controller) for internal and external daisy-chains. If then the internal ribbon cable is much shorter than the external 'system' cable, the internal cable may act as a 'stub' rather than as a branch, thereby having the disconcering effect of a terminator within that daisy-chain.
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