5.9 is the is the limitation that 99% of drives will run into if they are running a standard SATA drive. You can slightly higher if running an optimized hardware RAID 0 setup and the drives and controller have a good amount of cache (my setup got 6.1).
To get higher than that, you will need to step up to SSD over SATA or SCSI, iSCSI, or SAS.
Sorry guys, but 5.9 is it unless you make a significant increase in spending.
-Skater
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