I had a Mac Plus with an Apple HD 20 hard drive.

It wasn't internal but it was made for the Plus.

It was an odd beast that connected to the external floppy connector.

I also had a Mac Plus with an external 20MB hard drive (this was late 1988). It wasn't the Apple HD20 [0] (which I had never heard of until now) but a 3rd party model, it used the Plus's SCSI port and had the same footprint so it sat underneath. I see Apple also made an HD20SC that used SCSI.

I didn't understand walrus01's comment to mean an external, rather than internal, hard drive.

[0] https://en.wikipedia.org/wiki/Hard_Disk_20

Yes, but it was the first model with a SCSI port and commonly sold with an external HDD that it sat on top of.