Mac OS X: My (removable) disk appears twice, how do I fix that?

A double disk is visible when:

the disc was closed, removed or otherwise disconnected (while the drive is in use), and then re-connected.
the computer was turned off suddenly.

Disconnect the drive
Stop the CrashPlan backup machine (in terminal)
$ sudo launchctl unload /Library/LaunchDaemons/com.crashplan.engine.plist
Now go to the folder /Volumes in terminal
$ cd /Volumes
Make sure there is nothing on the undesired double disc (ending in "1")
In this example we assume that the name of the double disc "Data" is.
$ sudo ls -lat Data 1
Is there something there? If it is unnecessary, delete it. Or move that folder to another location, so it does not interfere when the disk is reconnected.
Example:
$ sudo mv "Data 1" /Users
Now connect the drive again. You should now see that there is only one indication of your disk.
Go ahead and start the CrashPlan backup machine again.
$ sudo launchctl load /Library/LaunchDaemons/com.crashplan.engine.plist

That was it.