If the CrashPlan app reports that files are "missing" from your backup selection, it is most likely because the files and folders have moved, or because a drive is disconnected or recently replaced. This troubleshooting article describes how to ensure that all of your files are backing up, regardless of whether or not they have moved.
Commonly, folders or files that display as missing have simply moved to a location outside your file selection. For example, if you were previously backing up files stored on an external drive that failed, then restored those files to a new location on your computer, you may need to add the new location of the files to your backup file selection.
When files move to a location outside of your backup file selection, it's important to add the new location your backup file selection and leave the original location selected until CrashPlan compares the already-backed-up files to the files in the new location (a process known as de-duplication).
Read our tutorial on moving files to a folder outside your file selection for full details on adding the new file locations to your backup selection, and options for removing the old location (after de-duplication occurs).
If an external drive is missing, but the drive is still connected to your computer and powered on, try initiating a file verification scan to see if the CrashPlan app detects the drive:
If the verification scan doesn't detect your drive, check to make sure that the drive name or letter hasn't changed. If the drive name or letter has changed, refer to our Rename External Drives troubleshooting article.
If the file or folder has been deleted from the computer (and you didn't want it to be), you can restore the deleted file to its original location from the Restore tab.