Do you have show hidden files and operating system files checked in windows explorer? There is always some overhead for the file allocation table.
So is this "J" a partition of a larger drive? I appear so. If we saw a screenshot of the entire drive it may help.
You can also try running WinDirStat and see if that reveals anything.
I should ask what OS are you running and 32 or 64 bit?
Now if you care to try and learn about file allocation indices, get yourself a cup of coffee or tea and have a read:
How much space empty directories take? In Windows and NTFS file system it say '0 bytes' which is impossible. How much is really taken up by a directory? Does it depend on the file system or oper...
superuser.com
Which is why that's not such an easy answer.
Lastly if you want to try something different I always use "EaseUS Partition Master" to manage my disks and flash memory. It does get around some of the odd things Windows will do to drives and partitions. You can delete and recreate the partition since you haven't written any data to it.
I wouldn't lose any sleep. It's not a rootkit, they live on the root drive (usually "C") so no worries there.
Let us know if you find out anything earth shattering, you never know.
