information code 4160

My scheduled incremental backup for last night did not successfully run, citing information code 4160 (about which I can find no info on the website). There was very little info, and in the absence of a backup log (which would be very helpful), I have no idea of the problem. I successfully ran a full backup a week ago to the same destination location on an external drive. My OS is Windows 7 Home Premium 64-bit SP1.