Platters keep on spinning
Flash may be the next big thing in the storage world, but hard drive makers aren't ready to give up on spinning platters just yet. Mechanical designs may not be able to match SSD seek times, but on the capacity front, the traditional model blows flash out of the water. A 64GB SSD just doesn't look that impressive next to a drive that packs 320GB into the same form factor. And when you're in the market for storage, capacity is rather important.

Seagate is an elder statesman in the mobile world, with fourth-generation 5,400-RPM and third-generation 7,200-RPM Momentus drives widely available on the market. Western Digital, on the other hand, is a relatively new player, having only been in the mobile space for a couple of years. Like Seagate, WD has drives at 5,400 and 7,200 RPM—the Scorpio Blue and Black, respectively. Some hard drive makers also offer 2.5" models with spindles spinning at 4,200 RPM, but they're painfully slow and generally only available in base configurations from notebook vendors. You'd do well to avoid them.

Drive manufacturers are rarely consistent when it comes to which performance specifications are published, so it's difficult to draw too many conclusions based on claimed data rates, especially when there are such large discrepancies between sustained and maximum speeds. Curiously, neither Seagate nor Western Digital publish seek times for their 7,200-RPM drives, yet both offer up seek times for their 5,400-RPM models. Fortunately, we can measure seek times and transfer rates ourselves, and we will in a moment.

However, before diving into our benchmark results, there are a few predictions we can make based on the basic drive characteristics detailed above. Seagate's Momentus 5400.4, for example, is likely to be overmatched here. The drive's 125GB platters have a lower areal density than the 160GB platters found on the other drives, which gives the drive head access to less data over the same physical distance. The 5400.4 is also stuck with an 8MB cache, while the rest of the drives have 16MB of memory on board.

So the 5400.4 isn't likely to be the quickest drive in the bunch, but it does benefit from the five-year warranty that Seagate extends to all its internal hard drive products. Western Digital's Scorpio Blue is only covered for three years, although as the company's premium notebook model, the Black gets five years of coverage. Longer warranties don't necessarily guarantee better reliability, of course, but you're at least entitled to a replacement drive for longer.

While we're discussing reliability, it's worth noting that only Seagate publishes a Mean Time Between Failure rating for its mobile drives, and then only for the Momentus 7200.3. The drive is rated for an MTBF of 500,000 hours, which falls well short of the SSDs we're looking at today.

Because the moving parts inside mechanical drives are prone to damage from physical shock, both Seagate and Western Digital employ mechanisms to move the drive head off the disk during idle periods. This capability isn't available on the Momentus 5400.4, but on the 7200.3 it's coupled with a free-fall sensor that can retract the drive head in just 0.3 seconds. Western Digital plans to offer a version of the Scorpio Black with a free-fall sensor, also, but it's not available just yet.

What about the best of both worlds?
There are clearly benefits to both the solid-state and mechanical approaches to notebook storage. If only a disk existed that combined the best of both worlds: high-capacity platters paired with a fast-seeking flash cache. A hybrid, if you will.

Hybrid drives were hyped a couple of years ago when it was revealed that Windows Vista would support such configurations. However, now nearly a year and a half after Vista's launch, hybrid drives are few and far between. Seagate does have a hybrid Momentus 5400 PSD listed on its website, but the drive has only 256MB of cache, a maximum capacity of 160GB, a relatively slow 1.5Gbps SATA interface, and a pokey 44MB/s internal data rate—hardly impressive specs. A next-gen Momentus hybrid is expected later this year, but there's no word yet on what we can expect from it.