I recently went through the upgrade process verbatim from Mint 19.3 (Tricia) to Mint 20 (Ulyana) and everything seemed to go as expected, I did have to re-add some services and such that were outside the standard services. However, the biggest issue I'm now encountering is that I had a volume group of 2 external HDDs under one logical volume, after the upgrade one of the physical devices is not being read and causing the mount point to fail to read any data whatsoever.
How do I begin to troubleshoot this? The last backup of the LVM was in July. When run gparted I get a bunch of errors for the device in question and Disks reads the disk as having 4.1GB when it should be around 3TB. I'd be happy supply any output from the console if that would be easiest, just let me know what would help to solve this.