Follow up: I solved my own problem, and thought I'd share this info here in case anyone else runs into this problem in the future.
After booting and selecting the hard drive in question, I ran the CONFIG command. Using this, you're presented with the option to turn certain features off. I did the following:
- switched SECURITY support off
- switched AAM (Automatic Acoustic Management) support off
- switched HPA (Host Protected Area) off
I'm guessing it was either SECURITY or HPA which was causing the problem, so if you're having a similar problem, I would try those first.
Once those features were turned off I was able to detect the drive parameters as usual and run ERASE and SCAN without problem.