5.9 is the maximum score for a disk under Windows Vista.
In Windows 7, they extended the score to 7.9 to accomodate SSDs, which are way faster then mechanical disks. Since you have a mechanical disk, you're still stuck at 5.9.
BTW the "Windows Experience Index" is a very coarse guideline, not a benchmark. It's not a good tool to use for performance testing.
You can get up to around 6.5 with hard drives, though it is hard (the 6.5 that I saw was with a couple 15k drives on a hardware RAID controller with a ton of cache). To get up into the 7+ range though, you do need an SSD. The score isn't just disk transfer rate - it accounts for access time as well, and SSDs are unmatched in that area.