|WinPE Phase (boot image)|
select disk <?>
attribute disk clear readonly
Replace <?> with the disk number of the disk (without the brackets <>). Save the Notepad file with the name diskpart.txt (for example). In the ConfigMgr console create a new package (without a program) with the txt file in it.
During the task sequence, immediately after the "Setup Windows and ConfigMgr" step, add a "Run Command Line" step with the command: "diskpart /s diskpart.txt" and use the package created before.
|Windows Phase after diskpart|
After that the disk will be online again. Hope it helps!
Source: Microsoft TechNet