PSA: Most updated drivers @ https://teac.jp/int/product/ud-501/download . Japanese/International site has the most up to date drivers including important fixes and Windows 10 compatibility. American site's drivers are behind in both categories.
silverbot01In addition to the previous PSA: me and a couple friends have confirmed the UD-501 (and possibly other TEAC UD-XXX DACs do not function properly unless you use a short USB cable. 3 feet seems to be a good length. Longer distances than 3 feet seem to create intermittent issues with a computer recognizing the device. Try to ensure the stress relief on the cable is of good quality as well.
Such a great DAC at such a great price on MD, these easy fixes from the driver to the shorter cord should keep great.