iPXE boot to Windows writing back to boot device

[SameGuyAsBefore]

Weird thing I can't figure out. I'm using a flash device to boot iPXE, if I boot to a linux system like the demo or CoreOS everything behaves normally. If I boot to a windows image / installation , somewhere between the switch from ipxelinux to windows the flash device gets written to. (am using http in ipxe to load the win boot stuff)

4 bytes get changed at 0x440 bytes in. Same place every time, slightly different data.

If I boot the windows image straight from flash, this doesn't happen.

It's possible the problem wouldn't normally be noticed but because my flash device is image trusted, I can't boot that device next reboot due to the data change.

Cheers,
Pete