Thank you, Maciej. Glad you solved it well enough for yourself. It’s been a few weeks since I was looking at that problem, so it’s a little hazy what I ended up with. My notes to myself say seeed support was no help at all, actually silent, sadly (seeed support has unfortunately been unhelpful, silent, or mildly confrontational so far, for me). Looks like what I did was install both windows and linux to the nvme, and I disabled emmc (well, at least, it is still disabled at this time). I suppose what I ought to try now is wipe the emmc so there’s not an OS there and re-enable it for data use. It was a little tricky as I remember installing the windows image as given by seeed to the nvme (perhaps a fresh install without using theirs would have been less trouble, but at the time I was trying to be as kosher as possible re the starting software and keep it as seeedlike as possible).
I notice I emailed myself about this, excerpted . . .
One basic problem was that if emmc is enabled, the bios really wants to boot to any windows on it, ignoring bootable content on the m.2 ssd [no matter what the boot order says in bios]. They have no answer so far for that (or even admit it is a problem).
Another problem is getting the windows 10 enterprise content onto an ssd in the m.2 slot.
I messed around for hours trying to play with (1) (no success), and eventually disabling emmc means it doesn’t try to boot from that, so there’s a workaround (or else, disable/enable emmc every time to avoid or use windows). I would rather not have to use such a workaround, and have a big ssd in m.2 slot, so try putting w10 on the m.2 ssd along with debian. Depending on whether I use windows much on this pc, that may not be needed, but I will leave the option open.
Re (2) I tried various methods to clone or migrate or restore the Windows 10 enterprise to the m.2 nvme ssd, but no luck, until, after I had finally and irritatingly messed up the emmc windows bootability, I was able to restore the emmc Windows 10 enterprise from the seeed Windows 10 enterprise image resource, and to restore as well (after disabling emmc) to the m.2 nvme drive, then shrink the partition on the m.2 ssd to allow for dual boot with debian until I figure out which approach I want to go (no windows at all, windows + debian dual boot, windows + debian as vm, but not sure I want win10 primary on this machine and not sure if one can access stuff like rpio with debian as a VM, so not sure about that last approach).
The restore procedure for solving (2) is, unzip the content of the seeed w10 resource, which I have stored as orig_w10_image__SD-JX-CJ41G-M-101-H. Then prepare a big SD card (I used 32GB) using diskpart, and copy the files from the resource to the root directory on the SD card. Then reboot to the USB drive/SD card, and windows will restore from it, critically making drive w: with the new w10 content, which is the drive you are restoring to and will be as big as the drive you restore to is (and I think it ditches any content on that drive).
The instructions for using diskpart to create the bootable usb with restorable w10 content [can be found by searching for this phrase and getting a hit on woshub.com (slash how-to-create-uefi-bootable-usb-drive-to-install-windows-7), look about one third of the way into the article], search phrase being “Using Diskpart to Create UEFI Boot-Stick with Windows”.