Many Lexar USB Flash drives can have their removable bit ‘flipped’ using the Lexar BootIt utility. I am not sure if Lexars more modern Win8ToGo flash drives work with BootIt though as I have not got one.
I your USB software needs to find AutoUnattend.xml on a removable drive, then a simple solution is to plug a small cheap removable USB flash drive into the system as well as the SanDisk ‘fixed disk’ flash drive - Windows will then pick up the AutoUnattend.xml from the small flash drive when it boots to Windows. Easy2Boot also supports this 2-drive method.
RMPrepUSB will work with USB Fixed disks - just press Ctrl-Z or use the Settings menu tab.
This trend of shipping USB flash drives as ‘fixed disks’ is getting more prevalent - manufacturers should provide a utility like BootIt to enable these drives to be converted to either fixed or removable. If not, then they are going to lose sales, receive returns and get many more complaints. They should also clearly state in the specifications which type it is shipped as. Having a ‘flip’ utility would increase sales and SanDisk should seriously consider producing controllers and utilitities which support this.