The actual Small Computer Systems Interface specification from ANSI states that
each END of the daisy chained cables are terminated. This indicates that the
HD20SC manual is 'incorrect' and the Apple SCSI Cabling System manual is
correct.
The diagram on page 15 of the "Hard Disk 20SC Owner's Guide" shows the
terminator on the extra SCSI port. On page 5 of the HD20SC manual, it shows
that when connecting a single HD20S to a CPU, the SCSI System cable should
connect the 2 machines, while the SCSI Terminator connects to the other port on
the hard drive.
The top diagram on page 12 of "The Apple SCSI Cable System" shows the
terminator goes between the cable and the (one) peripheral device. Page 10 of
the SCSI Cable System manual states that for the same configuration that the
System cable should connect directly to the SCSI Terminator and the other end
of the Terminator connects to the SCSI device (in this case HD20SC).
In truth, both ways work. However, if you were to check the amount of line
noise on both one-device setups, you might find a bit more noise on the
incorrectly placed terminator configuration.
The terminator should be placed on the SCSI input of the first device. If the
configuration has more than one device or more than 10 feet of SCSI cable
hooked together, a second terminator should be placed on the pass through of
the last device. If the user has a self-terminated SCSI device, it should be
the last device on the chain. You may refer to the Apple SCSI Cabling System
manual for all of the correct configurations.