Vista - Complete PC rejects my target drive

In working with Complete PC and wbadmin, I created a second volume to hold my
backups, drive E:. I made my first Complete PC backup to this drive
successfully. Now when I try to do another backup (which should be
incremental), drive E: is not shown as a valid target, even tho the backup
center in the control panel tells me my last back up was to drive E:. In
using wbadmin to try and start this incremental backup, wbadmin gives an
error and says the "location for backup is a critical volume". Now the only
other thing on this drive is the WinRE wim file and boot.sdi, which were
added after the first complete PC backup. I have tried renaming boot.sdi and
winre.wim but that did not help. I have found no info so far on what makes a
volume "critical" and how to work around this problem.