If you check the log file (X:\windows\temp\smstslog\smsts.log) which can be opened during task sequence with pressing F8 you will see the error:
The active system partition on a MBR system must be NTFS
Cause
This error might occur when you try to install a completely new desktop. Most hardware suppliers provide a pre-installation environment to install the OS. However, this pre-installation environment is present on a non-NTFS file system.
Solution
During task sequence, press F8 to open a DOS-prompt. Type the following commands:
diskpart
select disk 0
clean
exit
