Promise Technology today announced the availability of what it claims to be the world's fastest ultra-portable storage device with Thunderbolt connectivity: the Pegasus J2. At the time of this writing, the J2 appears to only be available at the Apple Store – $799 and $1499 for the 256GB and 512GB models, respectively. Techreport claims Newegg should have it for around $600.

The Pegasus J2 is indeed very fast, topping the 750MB/sec mark in sequential read benchmarks. Also, the form factor is indeed portable, measuring in at 2.91 x 4.33 x 0.81 inches and weighing just over 4 oz. However, the stiff price tag will undoubtedly keep most consumers at a safe distance, not unlike garlic and 1960s vampires. It's worth mentioning the device is only officially compatible with Mac OS X, so potential PC buyers will want to look elsewhere anyhow.

What makes the Pegasus J2 so fast? The device is comprised of two high-performance mSATA SSDs sandwiched together in RAID-0, giving it a fair amount of internal oomph. Thunderbolt, of course, handles this horsepower comfortably with its 10Gbps of theoretical bandwidth.

One area where the Pegasus J2 falls short though are random writes. Anandtech reviewed a J2 unit last month and discovered just how bad it is, with random write benchmarks hitting 200-400KB/sec. The reviewer blamed the unit's entry-level Phison PS3108 controller, but it's worth mentioning SSDs in general are notoriously poor performers when it comes to this particular discipline.

What makes the Pegasus J2 so expensive? Well, I'm not really sure – SSD prices have been plummeting as certain top-shelf 128GB models have bottomed out at just $70 (on sale, of course). And sure, Thunderbolt is part of it, certainly. Even a simple Thunderbolt cable is still $50 and said cables are almost entirely under Apple's control until 2013. Of course, the Pegasus J2 does not include a Thunderbolt cable, so there's one more thing to consider.

Although the price is undeniably stiff, it is the fastest external drive of its kind though – that'll surely be reason enough for some.