The great thing about removable storage is you can take it anywhere. Flash drives and even smaller 2.5" drives require nothing more than an empty USB port or two and voila - data at your fingertips. However both of these storage methods are still limited in their capacities and are significantly more expensive per-gigabyte than their full sized brethren. To have a truly mobile library, a more spacious 3.5" drive is required. Nowadays enclosures all fit the USB2.0 standard, but more and more are beginning to make use of eSATA as well. The latter is just as fast as internal SATA and provided the computer has an eSATA port, data transfer and storage can be done externally with the speed of an internal drive.