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Launch ScriptEditor and copy and paste this into the script window:
tell application "System Events" set visible of disk "NameofDisk" to true end tell
tell application "Finder" to quit delay 1 tell application "Finder" to launch
Replace NameofDisk with the actual name of your disk, hit the Compile icon to make sure the script didn't acquire any errors, and then hit run. This should set the visible attribute back to visible and quit and relaunch the Finder, thus making the change available.
The same thing can happen to other drives, both internal and external. You can use the same Applescript to fix those as well, just put in the name of the drive.