The HBC (the Plus) must supply termination power to its own on-board termination (resistor packs). Older Macs were not designed also to supply termination power to an external daisy-chain. The termination arrangements at the other end of the daisy-chain have to be taken care of by the user, ie the enclosure supplies 5V for the HDD's logic and 12V to drive the platters, but unless there are resistor packs on the HDD's logic board and a means of supplying them with 5V, the drive is suited only to a non-terminal position in the daisy-chain, or the enclosure has to provide for termination by way of a CN-50F socket into which a CN-50M terminator can be plugged. This latter case allows for other devices to be daisy-chained, of course.
Later drives provided optional termination and termination power, usually shown on a header as TE (enable) and TP (power) in, say, Seagate's lexicon, and less intuitively in Quantum's case.
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