I emptied my hard drive, but it still has 9GB on it

Well, there might still be a non-empty recycle bin. And the folder System Volume Information which (for system disks) contains recovery snapshots which can get quite large. Aprt from that I don't really know. It can happen, though, that the amount of free space isn't accurate due to some file system corruption. In most occasions this is harmless but running chkdsk should fix it.

In any case, if you want to install another operating system on it, I'd suggest you format the drive to start with a clean partition for the new OS. Then you also know precisely that nothing else is on there :-)

ETA: kquinn made a good point as well: The Master File Table (MFT) might get pretty large if you had lots of files on that disk. It takes up a little space + around 512 bytes per file. But for that to fill 9 GiB you'd have to have almost 19 million files. I know I regularly have a few hundred thousand but that much seems unlikely. Especially because all those files are probably not just 1 KiB each.

What could possibly be taking up 9.27GB of space?

The NTFS Master File Table typically reserves a bunch of space; 9GB sounds approximately right for a 465GB drive. Note that if you make a huge 464GB file, the MFT reserved space will automatically shrink (since that huge file only requires one MFT record -- the MFT reserves space for a worst-case scenario of zillions of small files, all requiring their own MFT entries).

Is there a tool to see what is going on?

Typically third-party disk defragmentation tools have some kind of 'cluster view' that lets you see exactly what's on the disk and where. There are also various command-line NTFS tools, but those can be extremely cumbersome to use on more than one or two files already known to be of interest.

Have you tried looking at the drive from another operating system? Usually the way they read file systems reveals more than Windows is willing to show. Linux live CDs are a good start.