Hi guys, I have noticed this for a while:
lh5.googleusercontent.com/--TtE ... mputer.png and found myself having to option explorer.exe to access the device.
Left is Windows Explorer. Right is Directory Opus.
It may be due to the device not reliably sending notifications when it changes between empty and non-empty. Explorer is more forgiving than Opus with such devices. (Something we may add a workaround for in the future.)
I expect that if you fully exit Opus and then restart it, the drive will be listed.
If it's a common problem, you can turn on Preferences / Folders / Virtual Folders: Show empty disk drives, and Preferences / Folder Tree / Appearance: Empty disk drives to make Opus show the drive regardless of its state.
Of course, if those things don't make a difference then my guess about the cause may be incorrect.
leo
Thank for the following settings:
If it's a common problem, you can turn on Preferences / Folders / Virtual Folders: Show empty disk drives, and Preferences / Folder Tree / Appearance: Empty disk drives to make Opus show the drive regardless of its state.
Above settings makes DO forgiving as Explorer 
Any news on a workaround? This problem is very inconvenient. I have two thumb drives and a card reader connected to my computer right now, and they don't show up in the tree or on my drives toolbar when I have them set to hide empty drives. I don't like showing empty drives because things become very cluttered with virtual drives, empty slots in my card reader, etc. Explorer shows everything as I expect, hiding truly empty drives and and only showing devices that are not empty.
Just a note - if I remove one of the thumb drives and plug it back in, then suddenly everything works right and all my non-empty drives show up. But on startup, Opus gets confused...