Free space on a hard drive has got corrupted, HELP

I have 2 8Tb drives that I manually keep in sync. Recently, I have no idea when, the free space on them just got out of step. I've JUST re-synced them but at this moment there is

1,416,419,140k (& 70,438 files) free on one and 1,938,941,052k (and 70,435 files) on the other. The difference of 3 files does NOT account for the almost half a terabyte difference.

I summed all the files and they are 5,808,282,200k on one and 5,778,681,337k on the other. The difference is because I have imaged my system (at different times) to each drive.

The one on the left (R: 1.4Tb free) is the one that appears wrong.

Two questions....

1) Why/How did R: get corrupted?
2) how do I fix the available "free space" on R: the incorrect drive.

Sorry, a thumbnail with the same drive somehow got attached to this question and I can't delete it.... Sorry

Are the clusters the same size on both disk as that could make a big difference

Both discs were bought at the same time and setup exactly the same. They ran forever identically but just differed recently. I'll try to paste a cropped Disk Management snapshot below?



I generally copy things to S: and once every couple of weeks sync R: to S:

I had considered RAID mirroring the 2 discs but, because my PCIe SATA card was new I didn't have confidence with it's RAID routines.

The discs have worked perfectly for about 19 months. I AM computer literate though not an expert. Have been programming for over 40 years :)

As you see there is about 6Tb information so I can't just format the disc and re-copy everything back to it. This is a solution I maybe forced to adopt!?

Thanks for your replies :)