Seagate has admitted its first-generation hybrid hard drives - standard hard disks with an added Flash memory cache - are not providing the promised benefits, but it blamed the discrepancy on PC BIOS and device driver software.

Seagate first discussed its Momentus 5400 PSD (Power-Saving Drive) line of 2.5in hybrid hard drives back in June 2006. At the time, it claimed the incorporation of 256MB of Flash would lead to big performance gains and power savings. Windows Vista's ReadyDrive system uses the Flash memory to cache frequently needed data so it doesn't have to spin up and access the magnetic storage so often.

But yesterday, at the start of the week in which the drives officially went on sale, Seagate coughed to the fact the 80, 120 and 160GB drives will not deliver the claimed "order of magnitude" improvement over standard 2.5in, 5400rpm hard drives, ExtremeTech reports.

"[Hybrid hard drive technology] is just not getting the orders of magnitude experiences that Microsoft originally touted," Seagate Product Manager Melissa Johnson told the site.

"There are issues both with the BIOS and devices drivers - they don't know how to utilize the Flash, she accused.

Seagate isn't the only hard drive company touting Flash cache technology. In 2006, Hitachi pledged 2.5in, 5400rpm hybrid drives for release in 2007.

And this past September, Hitachi too admitted hybrid drives weren't up to snuff.

The two company later joined Samsung and Toshiba to found the Hybrid Storage Alliance (HSA), an industry body charged with promoting hybrid drives.

Not much has been heard from the HSA since, possibly because of the issues Seagate's launch has exposed. The drive maker said the driver software was being worked on by Microsoft and HSA members.

"By the second generation, products will see the system benefits," Johnson promised, though she didn't specify when that will happen.