When clicking rescan on a storage respository in XenCenter, you may encounter an error like the one below.

The problem is usually due to a foreign file in the storage repository. To find out what the offending file is, turn to the power of the command-line. Login to the master server and run the following command.
tail -n 100 /var/log/SMlog

Review the log and find the offending entry. It might look something like this.
***** VHD scan error: vhd=/var/run/sr-mount/3219ad4f-242e-1744-6d38-78e17efdce69/9411b73c-7a3a-4812-b64b-edfe8f04ee70.vhd scan-error=-32 error-message='opening file'

In this case, the file /var/run/sr-mount/3219ad4f-242e-1744-6d38-78e17efdce69/9411b73c-7a3a-4812-b64b-edfe8f04ee70.vhd could not be opened. After taking a quick look at that file, I quickly recognized that it was a 0 byte file. It did not create properly. I moved it out to the /tmp folder and tried to rescan. This resolved my issue.