.INPROGRESS File Extension

Backup file created by Time Machine, the default backup program included with Mac OS X 10.5 and later; stores the backup progress of files as they're being backed up and is removed when the backup process is complete.

INPROGRESS files are created and deleted in the background without user visibility. However, in the unlikely even that a backup fails, there may be an INPROGRESS file left over. For the safety of backed up data, it is best to consult an Apple representative to fix this issue.

NOTE: INPROGRESS files are stored in the Mac OS X package format. Therefore, if necessary, you can right-click the file and select "Show Package Contents" to manually view the backup data within the package.