It might be already formatted, but in a different filesystem. Windows uses NTFS and FAT, any other is displayed as unknown type.
On the link you provided, it says the disk is recognized in one XP system. I would make a backup of the data from that HDD to another one, then I would run a checkdisk using the command:
chkdsk x: /f/r
where x: is the drive's letter on that PC. It might have some errors on it. Then, a clean-up followed by a defrag.