I believe unetbootin is looking for the USB drive as being Removable. Newer flash drives, for Windows 8 compatibility, are now configured as Fixed drives. Unetbootin needs to be updated to support the newer drives. In the interrim use RMPREPUSB, it is current and supported.
Two options; One getting Unetbootin to update their app, Two buy a different flash drive, either a SanDisk that is not Windows 8 compatibile or a different brand.
I suppose you could try returning the drive to SanDisk for a replacement and see if that helps.