Using winnt.sif with the 32 bit version of Windows it is possible to use

[unattended]

UnattendMode=FullUnattended

OemSkipEula=Yes

OemPreinstall=Yes

TargetPath=\WINDOWS

UnattendSwitch=Yes

WaitForReboot = No

DriverSigningPolicy = ignore

NonDriverSigningPolicy = ignore

ProgramFilesDir = "D:\Program Files"

to change the Program Files Directory to partition D. There is no mention of this switch in the 64 bit version for winnt.sif.

Is there a way of changing the registry entries from "ProgramFilesDir=C:\Program Files" to "ProgramFilesDir=D:\Program Files" before runonceex runs to install apps so they go to the D:\ partition?

All attempts at getting regedit to add registry entries with updated drive letters has failed to make any difference so far. I don't know at what point in an unattended install to do it at.

this works

ProgramFilesDir = "C:\Program Files"

CommonProgramFilesDir = "C:\Program Files\Common"

ProgramFilesDir(x86) = "C:\Program Files(x86)"

CommonProgramFilesDir(x86) = "C:\Program Files(x86)\Common"