Almost have a Compaq Portable 386/20 working, but floppy drive is acting up. Help?

[wowbobwow]

I bought this beautiful, very-clean Compaq Portable 386/20 about a year ago, but as of now I've never gotten to actually use it. Like most of these early Compaq machines, this one cannot boot if the BIOS settings are wiped, which they always are because the ancient motherboard battery is long dead. Unlike later PC compatibles, these Compaq 'lunchbox computers' don't have an onscreen BIOS system, and instead require that you boot from a special "DIAGNOSTIC" floppy to reset the hard drive settings. I've got a correctly-formatted DIAGNOSTIC disk, but the floppy drive just won't boot from it (and/or my 'known good' DOS 6.22 boot floppy) - the read-heads will sometimes scoot back and forth, but the rotation of the disk itself feels 'chuggy' and sluggish, like my old VW when I was trying to coax it up to highway speeds.

Here's a quick video showing exactly what's happening. I would love any tips or suggestions here - I'm excited to get this machine booting!

 

[rimmeruk]

Sometimes capacitors do not leak but dry up internally and show no visual external signs of failure. If the drive is over 20 years old then it's a recap for certain.
I'm not saying that is the cause of the fault, just something to consider. Next, I would try another cable to rule that out, but I guess you have already
Give the motor a spray of contact cleaner, the motor could be failing.