Just now I've uncovered the erase partitions option. That was the trick.

The host is booted off a USB drive; deleting partitions (skipping the USB drive of course) cleared out all but one SATA disk on each host. That SATA disk has a vmkdump on it that cant be deleted even though the vmkdump device is set to the USB drive. So I'm working on that for now.

[later]

Aha! It was configured as the scratch partition somehow, which is not supposed to happen in a USB-booted host AFAIK.