Freshly-dumped NTFS partitions can suffer corruption on
Windows shutdown. Subsequent re-starts trigger
chkdsk, which fixes the disk but loses some or all of
the dumped data.
The fact that this problem only occurs when the PC is
SHUTDOWN, rather than RESET, suggests that Windows is
updating NTFS management files with cached data from
the disk's pre-dump state. To be investigated.
This does not happen if the PC is RESET, ie the Windows
shutdown procedure is bypassed, immediately after the
dump. Subsequent shutdowns/restarts can be done as