But I found out that if I leave the installation disc in the CDROM-drive when I start the computer, the first I get is two screens of which the second lets me chose either Grub or Windows. And they both work!!! So now I can start both Linux and Windows in this way.
But I wonder if this is the only way - I would prefer to avoid using the installation disk. By the way, the computer starts with the installation disk even though the CDROM is not the first on the start-up list in setup.
I found that I made a mistake and did not install OpenMandriva in "UEFI mode" (thanks to @wizardfromoz for the mention of that to make me check it). So while I first thought that the grub bootloader was not installed... it is correctly installed now. But it still has issues... it still boots Windows directly. I'll look into that further.
If you haven't already... you will really need to disable "hibernation" in Windows 10. If you haven't disabled it, then follow these instructions below after restarting Windows (not a shutdown and cold boot... that puts it into hibernation).
You may can boot Linux without the CDROM if your Acer responds like my HP. Try this:
1. Use your F12 to get your Boot Menu.
2. Choose Boot from EFI File (not the OS Boot Manager)
3. It should be just a single entry highlighted that starts with NO VOLUME LABEL... just hit Enter
4. Next is <EFI>.... just hit Enter again
5. Arrow down to <openmandriva> and hit Enter
6. Arrow down to grubx64.efi and hit Enter (don't use the other one).
This should give you the screen you see when you use the CDROM... so you can choose OpenMandriva or Windows. Of course, you didn't need to do all that for Windows!


I really hate Windows 10 (and UEFI too). When I re-installed in UEFI mode, it actually all worked exactly as it should... booting to the grub screen where you can choose which to boot. But after just a bit of fiddling around it was back to booting directly to Windows. I think (and I've heard) that this problem may return.... especially, say, after a Windows 10 update (or other system changes). So when we get to some final fixes, you may want to write them down.... even the steps I just listed above, if they work for you. It's just a band-aid fix for now.... next is to make grub the default bootloader again, instead of Windows.