No, according to diskpart, there are no other volumes or partitions on the disk.
This disk used to be my C: drive, which is why, I'm guessing, the existing partition is the System Reserved partition. Could that somehow be the source of the issue? I don't see why it would be, but I'm not knowledgeable about this stuff. I think, though, that I've made this process work in the past in similar cases where the outboard HDD had been the C: on another machine.
chkdsk /f finds nothing wrong with the disk.
So I'm now a bit confused. Are you saying that, given that chkdsk finds no problem with the disk, it's now worth trying a third-party utility in the hopes of finding a problem with the disk that chkdsk doesn't?
My gut, for what it's worth, is telling me that this is not a problem with the disk itself. I think there's some problem with way I'm trying to mount it. I'm tempted to also say that maybe there's some incompatibility between OS, driver, controller, and/or disk but, given that this disk used to be my C:, I don't think that's it.