Public Service Announcement: if a Time Machine backup fails, it will leave behind a file with an extension of
inprogress. (It’s actually a folder.) If you have a hard time deleting it, do not use
rm! It will erase files from all backups, instead of just the one in progress. Instead, trash the
inprogress file from the Finder and Empty Trash as usual. It might take a while, but it will Do The Right Thing.
You could also leave the file alone and just try to backup again; Time Machine will clear out the failed
inprogress file when the next backup succeeds.