Symptoms: When 'mounting' the external disk, shared over the AirPort Extreme WiFi network, it is not added to the Finder's sidebar; instead, you get an error like this:
can't be opened because the original item can't be found.
Also, dragging the disk icon from the SHARED section in Finder's sidebar to the DEVICES section is not possibe; the Finder just displays a "can't drop it here" icon.
Searching online, I found a discussion on Apple's forums which contained the solution.
Fixing permissions with Disk Utility was not an option since a network-shared disk is not accessible within the app.
Apparently, the disk acquired an "alias" attribute by mistake. This can be checked in the Terminal, where the alias attribute is the capital 'A':
% /usr/bin/GetFileInfo /Volumes/AirDisk directory: "/Volumes/AirDisk" attributes: Avbstclinmedz
You can use the following command to turn the alias attribute off (then check again with the above
GetFileInfo command and look for a lowercase 'a'):
% /usr/bin/SetFile -a a /Volumes/AirDisk
After that, the mounting problem disappeared.