There may be some old files that can be deleted.
You would need supports advice on deleting these though.
We usually ask for a directory listing for review.
Open a command prompt and navigate to the Netflow\Datafiles directory
Then run
dir /s > datafiles.txt
Provide the file to support for review.
Sometimes there are older files that will no be processed that can free up some space but as Chris stated the application probably had a crash of the Reaper and the system may no longer be processing files.