CrashPlan app reports drive as "missing"
This issue occurs when the external drive is disconnected from the device or the letter or name assigned to the drive by your operating system changes. CrashPlan uses absolute paths when backing up. If the CrashPlan app cannot detect a drive at the specified location, such as when it is disconnected or when the name of a drive changes, it lists the drive as "Missing". You can correct the issue by reconnecting the drive or renaming the drive to match the original drive letter or name.
External drive stops backing up (OS X)
If an external drive stops backing up on OS X, it is likely because the drive didn't cleanly unmount (for example, after a power outage, or disconnection without ejection). When this happens, the drive can leave behind a "ghost" folder in /Volumes, even though the drive is no longer mounted. When the drive re-mounts, your device detects that the folder name for the drive is taken, and consequently appends a “1” to the drive name. However, CrashPlan looks for the folder with the original name, which is no longer connected. Use our troubleshooting guide to diagnose and correct the issue.