Your backup is able to get to a high 90+% but it never seems to get to 100%. In the History tab you see the message "Unable to back up x (number of) files."
There are likely files that CrashPlan is having trouble accessing. Often they are files that are in use by another application or they have limited permissions.
You can tell exactly which files are not being backed up by looking in the CrashPlan logs (specifically the file named backup_files.log.0). The logs can be found in in these locations:
Windows Vista/7/2008 - C:\ProgramData\CrashPlan\log
Win XP/2003 - C:\Documents and Settings\All Users\Application Data\CrashPlan\log
The backup_files.log.0 can be opened in a text editor. CrashPlan has processed, but any files with an "I" to the very left were backed up successfully, and those with a "W" to the left were files CrashPlan wasn't able to back up. If you’re backup is currently hovering in the high 90% range, then most of the files CrashPlan is having trouble with will be towards the end of the backup_files log.
The culprit is usually (but not always!) lingering VSS shadow copies. When programs try to access open files, the Windows Volume Shadow Service will pause the program writing to the file and create a snapshot of it to allow another program (like CrashPlan) to access it. Sometimes these snapshots don’t get deleted after use, and VSS limits the permissions on those snapshots, so CrashPlan can see them, but can’t actually back them up.
Instructions for deleting shadow copy files can be found here: