Hrm, I actually just ran df, and this is what it outputted. I guess I incorrectly assumed that what the debian GUI reported as free was what df would report. So why the discrepancy between the GUI-reported free space and what's shown here? Is this another 2^30 bytes vs. 10^9 bytes conversion thing? Also, what is taking up 73.352 MB of drive space? The only thing currently on that drive is the empty "lost+found" file.
This was the relevant output of df:
Filesystem 1K-blocks Used Available Use% Mounted on
/dev/sdb1 961432072 73352 912520720 1% /media/TBdisk
P.S. What would I do with Tune2fs? I formatted the drive with just default settings (no extra params).