Check for disk activity?

That may be in part because not all disk interfaces have programmatic control over the disk motor.

A WD1770 or 1772 can handle it automatically, with a delay for spin-down, just based on whether it's being asked to do anything. That's an extension over the older FDC1793 they're otherwise compatible with. So a disk interface might opt to use the newer and more abundant chips but thereby be unable to offer programmatic motor control.

So: not purely a software problem.

I guess the author is going to need to hack it, there being no standards-compliant way to achieve the objective. Maybe just count for some absurdly long amount of time, e.g. a whole minute, and assume the disk motor must be off?