I will assume you are converting a Windows based OS, so that an NTFS volume(s) exists.
That said, I found a nice little discussion in the VMware communities here:
To summarize, there are three issues to watch for:
– make sure the volume is clean (i.e. run chkdsk on the physical machine first)
– make sure no other program is trying to use Volume Shadow Copy Service (like a backup program? kill it)
– make sure the Volume Shadow Copy Service can start (there is a nice little batch file to force an XP machine to start manually, 2003 sb no problem)
Good luck. . .