Empty HDD showing 500 GB of data in Explorer

Hi guys,
I got a new external hard disk (M:) to keep all my backups on. Last night I transferred (by cut and paste) all my files from my internal backup drive (F:) to the new one. The cut and paste went fine and all the data were moved.

But my internal drive still shows 500 GB of data, which is not really there. There are no hidden/system files except Recycle bin and System Volume Info, as usual. The recycle bin is empty. See the screen cap below (drive F:).

When I look at it with WinDirStat it also shows nothing present. I ran CHKDSK - no problems. Disc manager also shows no problems and nothing out of the ordinary. A reboot didn't help.

I should mention, I formerly used this internal disk for my system backups with Windows Backup. They were also moved to the new disk (I know, they may not be reliable after being manually moved, but I'll make a new system backup to the new HD ASAP).

Should I just reformat the disk and be done with it?

Thanks for any help.

Keith