It's strange as the app doesn't do anything strange with sdcard (only use normal fs functions) so if it doesn't work it means that also other file saving homebrews should have the same problem;
some things you may try:
-Check the sdcard for corruption
-Check that there's no phisical lock to the sdcard (unlikely)
-Check that apps on your computer (like antivirus) doesn't do strange things with the card (for example Panda AV do a "vaccination" to drives) (unlikely)
-Check that contacts of both SD and Wii U sd socket are clean
-Try to format the sd trying different cluster sizes/other configurations
-Make sure the SDCard is fat32 formatted
-Do a total cleaning of the sdcard (with diskpart)
-Change sdcard
Alternatively it could be because your wii u can't reach the wiiibru server and leaves the download thread open in background (unlikely)