Does the Mac boot from the external FDD A9M0106 with the same disc or other known good bootable discs ?
If not, slide the metal cover of the floppy disc away and look with a magnifying glass to the surface while gently rotating it: do you see any scratches, even minor one's ?
It is possible that the external FDD was faulty and damaged the FDD controller or the Bourns filter.
You can also try swapping the drives and see what this gives. The drive in the external enclosure should be the same as in your Mac Plus.
My first shot would be cleaning the heads of the internal floppy drive with isopropyl alcohol and see what this gives.
I listened to the youtube video but the sound is very silent: it looks like the heads don't attempt to read the disc at all.