I recently had a drive go to "Disk is Missing" in the DrivePool(22.214.171.1244) dashboard on Windows 10 Build 1909. The disk appeared fine in Scanner, no SMART errors, showed up in Disk Management, files were present in Windows Explorer. The disk was mounted into a directory (vs a drive letter)
No idea what precipitated the error -- although a couple weeks ago, I removed a bad drive and replaced it with a refurbished one. The error showed up upon a reboot, but not the first reboot after the drive replacement. No UltraDMA SMART errors, like you might expect with a questionable connection.....in case that was in play.
I upgraded to 126.96.36.1999 and clicked Remove on the disk. This forced a Measuring.....then a Checking..... and then all was good. But never presented options to remove the drive, nor did it actually remove the drive.
I think whatever process evaluates it as missing was failing to detect it again. I tried unmounting then re-mounting via Disk Management, that did not work, the disk still showed as missing. I've also noticed that the emailed drive notification always seem to be too late -- I already know there's a problem before the notification is sent. I'm also not sure when that's evaluated, but it seems almost require an application restart, which seems odd to me.
Thought I'd share in case anyone else is seeing.