Well, I tried smacking the drive and that did indeed allow it to spin up.  I then tried reconnecting it to the Mac Plus, and it starts to load but ultimately fails.  Considering I witnessed copious amounts of smoke coming out of the Mac Plus, I'm thinking the problem is probably on that end. 

Also it's clear that I connected the SCSI cable to the wrong port on the hard drive, so I don't know if that is what was responsible for burning things out, or whether it just happened because the computer is nearly 30 years old.