Please note that if you have questions regarding the Windows Experience Index I suggest you to contact the operating system manufacturer since this is part of the (Os), a general recommendation could be to update your bios, SATA drivers and update the firmware of your Solid state drive if needed.
Microsoft artificially caps the Windows Experience Index hard disk subscore to 5.9 for all mechanical (rotating) hard disks. Only SSDs can score higher than 5.9. A score of 5.9 suggests that WEI may be confused about the identity of the SSD in the system. Does the system have both an SSD and a hard disk attached?