iPXE boot to Windows writing back to boot device

[mcb30]

(2014-03-10 10:55)robinsmidsrod Wrote:  I think this might be Windows trying to write a disk signature to the device. If you try to make a manual syslinux boot device and allow windows to write the identifier and THEN add ipxe.lkrn then you might have something that doesn't change on each boot.

From memory, Windows will always write data to the boot disk during a boot attempt. It does this in order to be able to detect failed boot attempts: it sets a flag somewhere on the disk before handing over from the bootloader stage to the full OS, and the OS clears this flag after loading everything. I haven't traced this through, but I'm pretty sure it's used to implement the "Windows did not start successfully" message.

I assume you're using wimboot, since you mention using iPXE to load Windows via HTTP. wimboot has no support for writing back to the virtual INT13 disk that it creates, and will always return a failure for any write attempt. It's just about possible that Windows responds to this failure by attempting to find another disk it can write to, and so writes to your flash device.

I've pushed a "fakewrite" branch to the wimboot repository which will ignore and report success for any write attempts. You could give this a try and see if the changes the behaviour at all:

Source code: http://git.ipxe.org/wimboot.git/shortlog.../fakewrite
Prebuilt binary: http://git.ipxe.org/wimboot.git/blob_pla...5:/wimboot

Michael