I think I may have found the reason for this. if you copy all (ctrl+A), and it's set to copy the system volume information folder (contains file system identifier) as well, it may lump some files into that folder. in my case, it was windows disk backup files. be weary of doing this in case the same thing happens to you. I was not able to figure out how to delete the files short of formatting the drive. as you can imagine, it's frustrating especially considering the drive in question was 10TBs, luckily I have several backups of the content including another 10TB drive, which I'm copying to now. it takes apprx. 3 days to transfer 8TBs (the amount of space being used) or so via usb 3.0 both ways. :-/
Used space would speak to either other files, or a different sector size.
Free space. The size on the thing is a bit of a mild suggestion for a lot of manufacturers these days. Obviously you have the counting in 1000 vs counting in 1024 thing that drive makers do but depending upon dead sectors and replacement ones available for allocation you might also get a few hundred megs more or less than another despite both broadly being the same nominal size.
actually, it's free space that's available that I'm looking at. two same make/model drives with the same data does not = same used space. the total space is the same by going into properties, but free space, by going into properties, and the space shown on my pc are different. I have a theory as to why this is happening. and, I think it has to do with windows backup. I believe it's creating large restore points in the system volume information folder which cannot be transferred correctly. I won't know for sure until I create a disk backup on one drive, then immediately create one on the second drive. is there a way to examine the content of that folder without admin privileges? and, can you create a backup over a backup or do you need to delete the original backup first?
after having done some research, apparently, the system volume information folder is also used for system restore points. I think it can easily be confused with a disk backup, since both are long strings of alphanumerics. the only way I was able to see the content was using treesize, which grants you privileges to open the folder, but then you can't close it back up again. this folder, more than others, seems very stingy in that it won't allow any tampering (i.e. no copying/cutting and no deleting). this is my thinking as to why the sizes are different. when you do a ctril + A over all content, it reads that folder and the windows imaging folders as 0KBs. however, copying and pasting the windows imaging folder, the true size is displayed. you can't do this with the system volume information folder. 0KBs = 0KBs then. I'm a little concerned that something isn't being read. :-/