Yeah, the free space amount is probably "overflowing" several times.
If it's REALLY old, could it only be a 32-bit count...? *shrug* In that case, changing the free space by as little as 4GB could make it come out "right" (positive).
Anyway, a the easiest way to make files of whatever size (almost 1.77 TB here), from Command Prompt:
fsutil file createnew C:\HUGE.txt 1900000000000
I guess it'll work that big!
I just tried a 400 GB one, which didn't take more than about 10 seconds (it doesn't write
that much data, it's just filling in NTFS structures or something for a file that big).