Western Digital has revealed they are sampling a new 5mm-thick (0.2-inch) hybrid storage drive that could end up in future Ultrabooks. Western Digital’s 500GB unit will utilize a small amount of flash memory in addition to traditional spinning media to offer customers an overall package that’s comparable to a dedicated SSD in terms of speed while retaining the capacity and cost benefits found in a standard hard drive.

Ars Technica notes that mobile storage solutions have traditionally been 9.5mm thick but here lately, manufacturers have been putting their drives on a diet in order to fit into increasingly thin notebooks. Samsung’s 830-series drives, for example, are now just 7mm in thickness – the same as Western Digital’s own drive found in the Acer Timeline M5 we recently reviewed.

A 5mm drive would require even less space which could translate into thinner notebooks but perhaps more interesting is the fact that these drives will use MLC NAND flash instead of SLC, a more expensive but more reliable type of module. Hybrid drives already on the market like Seagate’s Momentus XT use SLC and are typically limited to 4GB or 8GB of NAND flash. This means that Western Digital’s drive could see upwards of 32GB of flash memory onboard.

Finally, it’s worth pointing out that most “hybrid” drives found in today’s Ultrabooks actually use a smaller mSATA SSD for caching. This new drive from WD would feature the NAND and platters in a single unit, resulting in even more saved space.

We’ll likely hear more about this drive and others during the company’s Investor Day scheduled later this week on the 13th.