The only things I can think of are:
Poor Voltage spec when the external drive is attached. - Check/Verify/Adjust.
Miss-matched ROMs or older ROMs (shouldn't be in a MacPlus but - you never know what somebody else has done)
Miss-Matched Floppy drive in external case.
I don't know about the external drive BUT internally, there are 3 models of 800K drive and are used internally thus:
Series Number.................................Cable Needed
MFD-5lW-03 (Red on Silver Label)......Yellow Stripe (or Red Stripe with one cable on the ribbon cut open)
MFD-51W (Black on Silver Label)........Red Stripe
51W-10 (Black on Silver Label)...........Red Stripe
Finally, the most likely cause (back in the day) was a failed floppy port "Filter" - There are two involved. One filters the Mouse Port & Floppy, the other Floppy only.
In the picture below, the
left Filter does the Mouse and the ENABLE signal for the External Floppy Drive. It also does the Write and PWM for BOTH external & Internal drives, using the same signal path for BOTH.
The
right Filter does other signals for BOTH drives using the same signal paths.
In the circuit diagram, the filters are noted as U8A and U9A in ellipses.
I'd suggest checking everything else before trying to; a) FIND and b) REPLACE the filters.
A few manufacturers were used, but the easiest to find might be the Bourns 4120R-601-250/201
Anyhow, those are just my 'guesses'.