Removing USB mass storage devices causes volume auto-mount failure

I have a partition (volume label = "usr") which is mounted on C:\usr on my
system volume. This partition has no drive letter assigned; it is always
accessed through its mount point. There are a number of system required, user
specific files on this volume (e.g. Documents & Settings for my account). A
number of other partitions have mount points on the "usr" volume. For several
generations of Windows (Win2K, WinXP) I have used this configuration with no
problems.

I have now installed XP MCE, SP2, on a new computer, and am experiencing a
problem I've not encountered before: if I change the USB connected mass
storage configuration (i.e. unplug a USB connected hard drive, or my USB
connected Pocket PC) while the computer is powered OFF, the next time I boot
up the system "usr" fails to mount. On the other hand, if I make these
changes
while Windows is running, there is no problem on the next boot-up.

Any suggestions as to what is occurring? Any suggested solutions (other than
"don't do that!")?