The "FSGetMountSource failed" error means that the Wii U failed to open and mount your SD Card's file system.
There are numerous causes for this.
- The SD Card's file system may be corrupt or invalid (try using the SD Association's formatting tool and use it to firmat to the FAT32 file system using 64k clusters)
- The SD Card or the slot may be damaged/dirty - blow into the port and your SD Card, and perhaps try another SD Card to see if it works).
- Check that the SD Card is actually inserted into the console - it should click into place if enough pressure is applied.