So, some speedy USB sticks have SSD controllers inside. Some of these flash drives (USB 2.0 and especially 3.0) are seen by windows and marketed as external SSDs, some not (ex: USB 3.0 drives that have SSD controllers). And for some reason (I don't know and I don't care why, another Microsoft snafu if you ask me) the Autounattend.xml file isn't working if you boot from one of these drives... maybe Autounattend file isn't the only thing that won't work.
Tryed so far two of these drives (one Ridata USB 2.0 / eSATA SSD, one Corsair USB 3.0), and for both the Autounattend.xml didn't work.
Fortunately there is a way to detect faulty (from Microsoft perspective) drives: Windows 7 USB/DVD download tool - Microsoft tool for making bootable USB drives. As you can see here, that Microsoft tool can't find the Ridata drive, even after removing any MBR and formatting it both NTFS and FAT32 with a low-format software.
After I found an old flash drive that was ok with Microsoft tool, the setup became ~ completely silent with an Autounattend.xml I have, even if I booted and installed windows from a flash drive / USB stick. There are no problems with unattended when booting from a DVD, fyi.
LE: Other tools for installing windows from an USB flash drive like RMPrepUSB, UltraISO PE, WinToFlash and W7T integrated USB Boot Prep tool, although are very successful at making those SSD-based drives to be bootable, ANY Autounattend.xml is useless from these kind of drives.
Edited by RicaNeaga, 01 December 2011 - 01:27 PM.