This is because the recorders storage system only has a 255MB file size limit.
There are a couple options to fix this.
1. Lower the cameras bit rate so that it will fit the time you want to back up within the 255 MB file size limit.
2. Back up using the web browser this will allow for a 1GB back up file size limit.
3. Use the EZBackup software, this will allow for back up with no file size limit.