I didn't realize that the HD20 SC was a genuine external, sorry.  I thought you meant the SE's internal Miniscribe.

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https://68kmla.org/forums/index.php?/topic/10576-mac-plus-and-external-scsi/

The Plus lacks internal termination of the SCSI chain.  So you will need to terminate the external with a terminator on the back (I think?).

The internal drive on the SE terminates the SCSI chain, whereas without any SCSI devices by default the Plus does not.