[dexter1]

Just one question: Is the external SCSI terminator only required when you use a SCSI2SD adapter, as I run two Indy's without these external terminators without issue?

Please read Jan-Jaap's post about SCSI termination.
Basically, because the Indy (and Indigo1) only have one WD33C93 SCSI controller, both the internal and external SCSI bus-chains are on a single bus which needs termination on both ends. So yes Indy's and Indigo1 systems must have a terminator on the external bus.
You can possibly get away with not having a terminator if you only have a single device next to the SCSI controller, but i highly discourage it

In contrast the Indigo2 and O2 systems have two SCSI controllers, (two WD33C93 and two AHA7880 respectively), on for the internal bus-chain and one for the external bus-chain, so no termination is needed on their external connectors.