I assume you are using imagew as otherwise you would have to prepare the drive already if using imagex.
I have found the new version of imagew in SP1 (2.7.9) seems not to mount the volumes like the earlier version did. This means FixWindows will fail as it has nothing to fix so yes you do have to mount the volume.
The other issue is this seems that this is not consistent as I have had imagew sometimes mount the volume as the wrong drive and sometime no mount at all. To counter this I have now added a mount action followed by FixWindows and set both not to stop processing if they fail to cover both scenarios.
Also note that FixWindows and FixUnattend in SP1 are downloaded automatically to x:\ldclient.
