Why is NTFS disk corrupted?

Users get pissy when there is slow access to their data, so the OS compensates by opening the fileystem and caching data about the inodes inside to it's in a "ready to be accessed" state. If files are marked open/in-use and then the power is pulled, the filesystem is in an inconsistent state and will need to be checked and marked clean before it can be used again. Even if a system is functionally idle it's computationally always busy doing something (like checking it's still idle)..

Quote from LybsterKodi

I going to convert my 6TB HDD to EXT4 as soon as I free up a PC to install Linux on that's fast enough to transfer everything from backup to the reformatted disk.

You don't need to install linux for that. You could use a "Live linux" from USB stick. Tip: search for "Ventoy" (booting ISO directly from USB stick) and "Xubuntu". Just be carefull, "playing" with partitions could lead to data lost...