Update: I didn't get time to run chkdsk before the next backup. The next scheduled backup ran and completed normally. I went ahead and ran chkdsk with the switches you specified just in case. It ran successfully last night. It looks like everything is good to go for now.
Out of curiosity, is this a common occurrence? I had never had a backup fail since installing PP1. It was only one volume, of 4, that failed, which has Win7 os installation.