There are a number of different reasons why a Destination Unavailable message appears in CrashPlan:
This article explains causes for each of these circumstances.
“Destination unavailable. Backup location is not accessible” means that the location of the archive on the destination computer is not valid. This could be because the drive is not currently mounted or because permissions do not allow access. It is not a network error.
Using Applications > Utilities > Terminal, if you enter the following command:
Do you see the external drive name? Do you see one ending with a “1”?
If you have a folder ending with a “1” it means that for some reason the drive didn't cleanly unmount and left a “folder” in /Volumes. Then, when the drive re-mounts, Mac sees that the folder's name is taken so it appends a “1”. However, CrashPlan is looking for the folder with the proper name, which is no longer valid.
This message has two possible causes:
Both settings can be adjusted in the CrashPlan app from Settings > General. Increase the time period during which CrashPlan is allowed to run or increase CrashPlan's allowed CPU usage to reestablish back up to the destination.
The destination computer is currently performing Archive Maintenance / the Compact routine. The "Destination Unavailable" message will disappear once the maintenance process is complete.
Your friend has restricted the amount of space your backup can take up on his or her computer and you've reached your limit. Files are still available for restore, but no additional files can be backed up to this destination.
To continue backing up to your friend's destination, either adjust your file selection to make the backup smaller, modify your frequency and version settings, or ask your friend to increase the allotted space for your backup.