I am a non-expert in either Linux or PCs in general. I attempted to upgrade my dual boot Win 8/Linux Mint 19.3 system to Mint 20. I experienced a system freeze with no error messages and thing spiraled south from there. I attempted this foolishly after having successfully updated from 18 to 19.3 with no issues. After many repair attempts which only contributed to the downward spiral I did total Windows reinstall (restored HDD to initial factory configuration). Windows install was successful. I shrank the windows install to 50% 0f the 1TB HDD and attempted a dual boot install of Linux 20. I received "input/output error during read on /dev/sda". It seems Linux can not read the HDD and sees only 1 large partition. During the install process the necessary partitions are created for the install but can not be used. After several failed attempts with the same results I tried to install 19.3 using the install media I had used when I replaced the HDD last year. I followed the same process I had used then which worked but received the same errors I received trying to install 20. Investigation revealed the error "Libparted error, input/output error during read on /dev/sda, Primary GPT table is corrupt but backup OK so that will be used".
Has Microsoft pushed an update that makes dual boot impossible? A bad HDD is constantly suggested as the solution but I doubt that is the problem as checking the disk with various checking protocols shows no issues and very few bad sectors. Can the GPT table be restored from backup. Online searches seem to find fixing corrupt backup but not restoring corrupt primary from backup. Any help is greatly appreciated, but I am not an expert so please make any suggestions newbie doable. Thanks

To me this smells like it may be that Windows "restored" in GPT/UEFI mode, which creates a "guard" partition in front of the the real partition table, followed by an attempt to install Mint by booting the installation media in non-UEFI mode. I know a way you should be able to get this fixed, but it involves non-free software that I use for all partitioning, which I always do in advance of doing any OS installations. Surely there must be a free tool that does the same. I just don't have any familiarity with what it/they may be.

If it was mine I'd install Windows from an original installation media rather than a restore disk, after creating the desired a whole new set of partitions for GPT/UEFI OS installations. I'd also create two sets of Linux root partitions, one to keep on working when it's time for the next release, to make sure the new release is acceptable before ceasing to use the old, keeping the old for fixing anything that might break in the new, until time for the second new installation/release to replace the first.

BTW, Mint 19.x can be upgraded to Mint 20. I've done it at least twice, using the methodology for Debian upgrading to a new major release.

HTH

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Thanks for your help. There were several tiny "unknown" partitions on the HDD even though it looked like one large unallocated partition. I was able to delete, format and erase the entire disk using Killdisk on Live CD. re installed Windows 8.1 and then was happy to see the "Install along side of Windows Boot" option when I installed Linux mint 20. Everything running great now. I really appreciate you sharing your knowledge. Thanks again.

I just recently had a drive that gave me that same error for over 3 months but otherwise seemed fine. Then all at once it bit the bullet and was totally toast. I would suggest replacing the drive since the error is in the GPT table area and if that spreads it can kill access to the entire drive.

boot mint iso, open a terminal post the ouput of the following