I usually disable CSM along with Secure boot. If CSM is enabled it has the possiblilty of you installing in Mbr mode (Bios) as opposed to UEFI. In UEFI you should set the settings to make sure Secure boot is disabled, CSM disabled and set it to UEFI only. The only reason one uses CSM is to boot legacy.
Perhaps installing the bootloader to the MBR of the drive might work without chroot as well but I don’t think an unmounted drive is a must since you could do a grub-install in a running system in both UEFI and MBR.
doing this from the live session terminal just gives the error mount: /mnt: wrong fs type, bad option, bad superblock on /dev/sdd, missing codepage or helper program, or other error.
My bad! There is a typo.
It should be: sudo mount /dev/sdd1 /mnt
Edit: it’s getting late around here as well so I perhaps be better logging out and head to bed soon 
Hey Gnomes are supposed to be on guard 24-7 
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doesn’t seem like it’s helped at all, at this point i’m wondering if replacing grub with some other bootloader would be a viable solution
Taking a power nap is allowed thought!
Occasionally.
Their beauty sleep? I figured, it is all about cosmetica-ization !
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Not sure at this stage what might be going on.
@ricklinux, however, seems to have waken up from his plasmatic sleep 
He might be able to assist you further.
But I’m not that good like you are with these grub problems. 
Ah… come on!
I have seen you reviving many a system.
Not really too many. Can you actually boot a Bios system from a UEFI install? I always use an f key in that case? 
Edit: Doesn’t it require that you have a boot partition for the installed Bios system? Etc?
Right. But OP’s Linux is a Bios install.
You mean that CSM might be disabled in his firmware settings?
Not sure then how they would be able to boot up the live session in Bios mode.
Well it should be disabled to do the install otherwise that is what can happen. It unintentionally gets installed in bios mode because the user doesn’t know any different from booting on the ISO. So if the user already has it installed in Bios mode they would have to have CSM on to be able to boot it. I previously said it needed to be off but that’s if it’s not installed yet. I specifically said it’s only needed to boot legacy mode. So if it is off the OP needs to try it with CSM turned back on or install EOS in UEFI mode.
Edit: You will be able to boot it in Bios mode. Just not necessarily from the grub menu.
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@albert-softie
I’m not the expert when it comes to fixing grub issues. But here is a little explanation maybe that helps. I think you can boot a Bios installed system if it has the mbr partition and a boot partition. Normally when you have this happen like in your situation you can boot the legacy install via an f key. CSM would have to be enabled. in order to boot in legacy mode. Anyway read this info and hopefully it helps you understand some of it.
new development:
turns out i actually had CSM enabled this whole time, and since i rarely ever change bios settings, this means i had to to have enabled it at some point prior to installing eos for some reason. once i had disabled it, the only boot drives appearing in my motherboard’s boot menu were windows and the live usb. after re-enabling it however, the eos drive is still missing from the boot menu, and the only addiction is an ext4 formatted nvme ssd which has nothing installed on it
Your linux install is here.
/dev/sdd1 * 2048 3870985326 3870983279 1.8T 83 Linux
/dev/sdd2 3870985327 3907024064 36038738 17.2G 82 Linux swap / Solaris
Is that not the NVME drive Samsung SSD 970 Evo Plus 2TB
That’s what it looks like to me. So if you boot and use the f key to select this drive does it boot?
Edit: Is that where you intended to install it? That’s a 2TB drive which when partitioned and formatted would be 1.8 TB approximately.
Edit: The other drive is only 1 TB and from your output that shows Windows is installed on that disk.
From what i see below!
Disk /dev/sdc: 931.51 GiB, 1000204886016 bytes, 1953525168 sectors
Disk model: Samsung SSD 860
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: gpt
Disk identifier: 88958834-A1C0-48F4-8ABF-C731917F06A7
Device Start End Sectors Size Type
/dev/sdc1 2048 1023999 1021952 499M Windows recovery environment
/dev/sdc2 1024000 1228799 204800 100M EFI System
/dev/sdc3 1228800 1261567 32768 16M Microsoft reserved
/dev/sdc4 1261568 1952392089 1951130522 930.4G Microsoft basic data
/dev/sdc5 1952393216 1953521663 1128448 551M Windows recovery environment
Disk /dev/sdd: 1.82 TiB, 2000398934016 bytes, 3907029168 sectors
Disk model: Samsung SSD 870
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0x0d2e3293
Device Boot Start End Sectors Size Id Type
/dev/sdd1 * 2048 3870985326 3870983279 1.8T 83 Linux
/dev/sdd2 3870985327 3907024064 36038738 17.2G 82 Linux swap / Solaris
well no, in fact the fdisk output clearly shows that my eos drive is a samsung ssd 870, not the 970 nvme. i had gotten the nvme drive for christmas this year and only got around to installing and formatting it the day before eos stopped booting. as i said, nothing is installed on the nvme, so attempting to boot from it just results in the 0xc00000e error screen once again
welp, not entirely sure what fixed it but after looking at my motherboard’s boot menu again this morning i realized the eos drive was there, and i could finally boot into it. and after changing some bios settings, my pc now properly boots into grub as it should. thank you to everyone for putting up with me, in case it wasn’t obvious i’ve never had to deal with anything like this before, and hopefully i won’t have to again
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Great you were able to get it running again! 
If you ever again encounter a problem with the system, you know where to get help!