And here I thought I was familiar with every SysInternals tool...good find, Steve.
Note: you must run this utility as Administrator to see the drive information.
Surprisingly, it showed 5 volumes for my 3 disks. Surprising because none of my drives are partitioned. I'm guessing those mystery volumes are boot sectors plus that little space Windows reserves. My C: and D: drives are both bootable and show 3 volumes each. Drive E: is not bootable and shows 2 volumes.
If I pop in a USB thumb drive, it takes the name DR6. If I unplug it and plug it back in, it then takes the name DR7 (and volume 10). I'm guessing the number increments each time, which explains where my DR15 and DR17 references came from. This also fits in with the way USB devices are sequentially enumerated.
Thanks, guys, for helping me solve this little mystery.