You certainly would not want to operate a bare hard drive externally more than occasionally, but those occasions do exist. For example, after you build a new computer or decide to upgrade your disk, your old drive will likely sit somewhere on a shelf - and then you'll realize that you forgot to copy some files from it. When I notice these things, I'm usually in a hurry, so crawling under the desk to disassemble half of my carefully assembled rig is not an option. But the USB 2.0 to SATA/IDE adapter from Brando certainly is.
There are multiple motherboards that feature interface options to attach hard drives externally. While eSATA (external SATA) is the future standard for external consumer and entry-level business class storage devices, there are still many home-made solutions. These solutions are as simple as providing a Serial ATA connector at the back panel of a motherboard, so this connector can be accessed from outside the PC. Gigabyte is a good example: the firm has been good at shipping external SATA (not eSATA) cables with some of its motherboard products.
The problem, though, is how to operate a drive without a proper power supply. The usual approach is to buy two or three Molex extensions or y-cables (these are the 4-pin internal power cables), so an internal drive that is operated outside the computer can be powered. Having both the power connector and a Serial ATA interface available from the outside then allows you to attach a SATA hard drive and run it as if it were integrated into the computer. However, the wiring is messy and the conditions for a hard drive operated externally are far from ideal: there is no air stream to cool the drive, and it is mechanically vulnerable. If your dog knocks over an external drive that is running while sitting vertically, that will probably kill it.
From an operating environment point of view, no hard drive should be operated without proper installation, and we strongly recommend that you buy an external drive enclosure kit or an external storage product if you need mobility or flexibility. Yet there are some occasions when hooking up a drive for a couple of minutes is sufficient. Brando developed the USB 2.0 to SATA/IDE adapter for these situations, which allows you to run either Serial ATA or UltraATA drives over USB.