For all those who want to set/correct the drive letters before the installation starts here's the description:

The mount manager loads the mount points out of the setup-registry if the key "HKLM\SYSTEM\MountedDevices" exists. The hive used for the setup-registry is located in the "I386" folder and the filename is "setupreg.hiv".

To change the drive letters you simply have to add your mount points to the hive. Since every drive has got a unique ID the best way to set the correct values is to import the necessary keys from a running system.

Hmm so do we want the

\??\Volume{3b40f43b-fba7-11d7-aaf1-00055d302a42}

type entries, or the

\DosDevices\A: type entries?

Or both?

AFAIK you need to add both.

Oh not to forget - every volume has got 2 entries, a "\DosDevices" and a corresponding "\\??\Volume" entry. They're linked through their volume id !