Also, if any sort of 3rd party backup is running on the volume then also the issue can occur.

Make sure no 3rd party backup is running on the volume.

Issue:

Discover scans fail with numerous Unknown errors

FileReader.log contains:

SEVERE: Unmount Exception:
java.io.IOException: Unknown Error
at com.vontu.directorycrawler.VontuFileException.<init>(VontuFileException.java:160)
at com.vontu.directorycrawler.sharepointmapper.GenericSharePointMapper.unmapDrive(GenericSharePointMapper.java:311)
at com.vontu.directorycrawler.sharepointmapper.SharePointMapper.unmount(SharePointMapper.java:89)
at com.vontu.directorycrawler.AbstractFileSystemCrawler.unmountShareIfNecessary(AbstractFileSystemCrawler.java:267)
at com.vontu.directorycrawler.AbstractFileSystemCrawler.processContentRoot(AbstractFileSystemCrawler.java:212)
at com.vontu.directorycrawler.AbstractFileSystemCrawler.crawl(AbstractFileSystemCrawler.java:159)
at com.vontu.discover.crawler.AbstractCrawler$3.run(AbstractCrawler.java:113)
at java.lang.Thread.run(Unknown Source)
Jul 8, 2007 5:16:49 AM com.vontu.directorycrawler.AbstractFileSystemCrawler unmountShareIfNecessary
WARNING: failed to unmount:

and

INFO: Error output from umount command The device is being accessed by an active process.
More help is available by typing NET HELPMSG 2404.
Jul 8, 2007 5:16:09 AM com.vontu.directorycrawler.sharepointmapper.GenericSharePointMapper unmapDrive
WARNING: Could not identify command error condition!

Explanation:
Basically, two people can both remote desktop into a box and mount a share to l: without effecting each other. Each has a 'session'. Both can't mount 2 l: s in their own individual session. The Symantec DLP service basically maintains its own session and 'somehow' lost track of the unmount and therefore couldn't mount the new one.

Solution:
Restart the Symantec DLP Services and resume scan