Small storage is the trend nowadays and here's a review comparing four pocket HDDs. Bill from Personal Tech Pipeline had the chance to test the Archos ArcDisk, Imation Micro Hard Drive, Seagate Pocket Hard Drive and the Sony Micro Vault Pro:
The pocket pals reviewed in this article are from Archos (4GB), Imation (2GB), Seagate (5GB), and Sony (5GB). All of these drives are powered through the computer’s USB port. Don’t expect them to work from a non-powered USB hub or keyboard. Both of these devices draw power from your PC’s internal USB, where the maximum available power is 500 milliamps (mA). Adding a mechanical device like a hard drive may push you beyond that maximum.