|Flash drives are the least reliable of modern storage media. Add to that the possibility that the device may be lost, stolen, or physically damaged. These devices should never be trusted for primary storage of important files but should be used primarily as a transport media. Having the only copy of an important file on a flash drive is asking for trouble.|
All files of any importance, wherever they are stored, need at least one backup copy while those of particular importance need 2 or more backup copies.