Always use the internal drive to download/repair/extract and let Usenapp move the files to an external drive after the file is extracted.
If you are downloading to an external drive at high speed while a repair/extraction is also in progress, the USB/network channel may get full and the downloaded data cannot be written to the external drive and will be kept in memory until the disc is available again.
You can also try to disable ‘concurrent jobs’, but that makes downloading/processing files very time intensive.
Preferences->Download->Memory