Thanks for the suggestion, I'll give it a try. I'm not sure why it should make any difference, as running setup just creates the folders on C: and it uses those after that, so it shouldn't matter where the setup is run from but I've got nothing to lose by trying it
Also: WHERE you got that ISO from? I would strongly suggest to download the ESD and create the ISO using the Decrypter (esd_decrypter_wimlib_8)!
I think the problem is that on rebooting to continue Windows setup, setup cannot find the place where you saved the files. Note that when you change the boot order, the drive letters assigned by Windows also change. Try burning the iso to DVD or make a bootable USB pen drive. Now, from within Win 7, browse to the DVD or pen drive and run setup from there.
I see little point using grub2dos. Just use easybcd to configure the windows bootloader do dual/multiboot wit linux and other oses. But aside that the solution seem pretty obvious even if you want/need to use grub2dos Just restore the windows bootloader, finish the w10 installation then reinstall grub2dos.
If you like to use grub4dos, I suggest you temporarily disable it. The update process insert its reboot process inside BCD with various flags. Use diskpart as administrator, select your disk, select your partition where you want to update, mark active, go through the update process and re-enable your grub4dos. e.g. diskpart DISKPART> list disk Disk ### Status Size Free Dyn Gpt -------- ------------- ------- ------- --- --- Disk 0 Online 149 GB 0 B DISKPART> sel disk 0 Disk 0 is now the selected disk. DISKPART> list part Partition ### Type Size Offset ------------- ---------------- ------- ------- Partition 1 Primary 48 GB 1024 KB Partition 2 Primary 50 GB 48 GB Partition 3 Primary 50 GB 98 GB DISKPART> sel part 1 Partition 1 is now the selected partition. DISKPART> active use bootsect to overwrite the grub4dos bootsector, type bootsect /? for help.
I tried creating a USB Win10 stick and running setup from that, which didn't help and I still got the same error. Thanks for the information about it using the BCD. I'm not sure why it doesn't work with grub4dos as the BCD is loaded when I select which partition I want to boot from. I don't normally see the boot menu as I only have a single entry for each Windows but if I add extra entries I see it. I think the problem might have been that the Windows 10 partition wasn't marked Active. I should add MakeActive to each of the grub4dos Windows boot entries to avoid problems in future. Anyway, I'll try installing the Windows MBR temporarily and see if that works. I can do that with BootIce, which is easier than using diskpart and I can check the BCD with EasyBCD.