I can’t boot into Windows in any known - at least to me - way, that’s the whole reason I’m doing this 
When I installed EOS, Windows was working fine - I used to start it from the BIOS boot options. After fixing the broken grub and cleaning up my NVRAM entries I wasn’t able to find Windows anymore.
I’ll look into the link you sent about rEFInd, thank you.
Yes, rEFInd should be able to.
Quote: “rEFInd supports booting legacy OSes”.
But note: On UEFI-based PCs, rEFInd defaults to scanning for EFI, but not for BIOS, boot loaders. If you want to launch BIOS-mode OSes from rEFInd, you must edit the scanfor line in refind.conf, as described on the Configuring the Boot Manager page.
You can try editing refind.conf (usually in /boot/efi/EFI/refind):
Uncomment the scanfor line and add hdbios to the options. This will tell rEFInd to activate its support for booting BIOS-mode OSes.
You’ll probably get one or two gray diamond-shaped icons when you boot, one of which should boot Windows. (If you get just one icon and it doesn’t boot Windows, also try uncommenting the uefi_deep_legacy_scan option in refind.conf .)
Try again after changing refind.conf.
As an alternative to all of this, or if rEFInd’s BIOS-mode support doesn’t work on your computer, and after checking if Windows boot record is still functional by trying to boot into it through your BIOS …
…, you could try repairing/reinstalling the Windows boot record (see link in post #29 ArchWiki: Restoring a Windows boot record. A Windows install medium usually provides the tools needed.
After this try rEFInd again.
Other possible ways to fix:
Convert Windows to boot in EFI mode rather than BIOS mode. This is risky and I don’t have any experience in doing this.
Reinstall Windows in UEFI mode. This will most probably solve your dual-boot problems.
Your Windows has an active “hiberfile” in which (some of) the systems state is stored on shutdown to enable a faster (but “unclean”) boot. This is one of the tricks Windows uses to make you believe the boot process is really fast.
If you plan on permanently mounting your windows partition under linux you’ll have to disable this “fast restarting” feature under Windows.
You can still forcibly mount this partition by removing the hiberfile:
sudo mount -t ntfs-3g -o remove_hiberfile /dev/sdb2 /mnt/win
I already checked about hibernated Windows and found out about fast start-up. After trying to mount it anyway I get this:
$ sudo mount -t ntfs-3g -o remove_hiberfile /dev/sdb2 /mnt/win
Mount is denied because the NTFS volume is already exclusively opened.
The volume may be already mounted, or another software may use it which
could be identified for example by the help of the 'fuser' command.
So I checked with fuser, although I’m not very familiar with this command and:
$ sudo fuser -m /dev/sdb2
/dev/sdb2: 3321
And I guess Windows is in fact mounted already - the question is… where 
??
Reinstall Windows in UEFI mode. This will most probably solve your dual-boot problems.
I’m thinking of going this way, even if it worked fine booting alongside Ubuntu and EOS for a while in legacy mode, I hope this makes things easier.
Try findmnt.
And have a look at ntfsfix as well. It might help in some cases. In your case it is about that hiberfil.sys, but still. I recall having seen some case when it actually resolved that issue as well.
I don’t think it’s actually mounted. Have you tried rebooting and mounting the ntfs partition as your first action?
Did ntfsfix help out?
Have you tried to repair the Windows installation with Windows tools?
If you enter e. g. “master boot record repair windows 10” in your favorite search-engine you should find plenty of tutorials. You’ll need a windows install medium though.
.
If this works and you can boot into Windows from BIOS again, it should be easier to add a boot entry to grub or rEFInd.
If you don’t have a highly customized Windows install with data you haven’t backed up elsewhere it may be time to cut your (time-) losses and reinstall in UEFI mode. 
