Although most systems do not need a removable or external hard drive, such drives have the following uses:
Installing a larger hard drive in a desktop system is trivial, but on a proprietary laptop system it may be impossible or extremely expensive to upgrade the hard drive. If you're faced with this situation, using a removable or external hard drive may extend the usable life of the laptop. If the laptop has an available PC Card slot, install a USB 2.0 PC Card adapter so that you can take advantage of the much higher throughput of USB 2.0 external drives.
If you need to transport large amounts of data to remote sites, a removable hard drive may be the only practical option. Cartridge-based removable hard disk drives store up to 20 GB, and frame/carrier-based removable hard disk drives are limited only by the capacity of the largest standard hard drives available. For example, one of our readers works for a company that produces digital special effects for movies, always on very short deadlines. The time needed to back up 100 GB of image data to tape?as well as the cost and complexity of installing the required $35,000 tape drives at each end?makes tape impractical. Instead, they install a frame-based removable hard disk drive system at each end, copy the huge datafiles to high-performance SCSI hard drives, and FedEx the hard drives to the movie production company. When the disk arrives, the production company staff simply plugs it into the frame and copies the data from the removable hard drive to the server. Now that USB 2.0 external hard drives with capacities of 250 GB or more are available at reasonable prices, we expect them to be used for this purpose.
Software development and similar work that requires using multiple operating systems always presents a problem. You could configure one PC to multiboot different operating systems, but that is seldom entirely satisfactory. You could install a dedicated computer for each OS, but doing that is expensive, generates a lot of noise and wastes heat, and means you're soon covered up in computers. Or you could use one computer with a frame/carrier-based removable hard drive. Installing each OS on its own hard drive means you can simply insert the carrier with the appropriate OS, restart the system, and have the equivalent of a dedicated PC running that OS.
That's why we use a frame/carrier unit on our main test-bed system. We know one person who does the same on his home computer. He, his wife, and his children each have their own hard drives mounted in carriers. He uses Linux, his wife uses Windows 2000, and the kids use Windows 98. When someone wants to use the computer, he inserts his own hard drive and restarts the system to boot into his own environment, with no worries about conflicts or accidentally damaging someone else's data or configuration.
Although it is a specialized application, some scientists, market researchers, and others need to manipulate extremely large data sets?sometimes in the 100 GB range. Although it may be possible to build a PC with sufficient disk space to store all the data sets on the main hard drive, it's often cheaper and more efficient to allow those data sets to be swapped in and out as needed. If there are many such data sets, using a frame/carrier removable hard drive may be the best option.
Although one or another tape technology is usually the best choice for routine backup, there are times when tape is simply not practical, either because of the time required to back up or the time required to restore. For example, assume that you have a 150 GB database stored on a mirrored set of Seagate Barracuda 180 Ultra320 hard drives. Your backup window is very short, and no tape drive is large enough and fast enough to back up that entire 150 GB in the time available. Furthermore, even if tape were workable for backup, you can't afford to have the database offline for the time it would take to restore it.
If the frame/carrier unit, the OS, and the host adapter all support hot-swapping and RAID, you can remove and insert drives as needed without downing the server. In that situation, you might install three frame/carrier-based removable hard drives, each with a Seagate Barracuda 180 hard drive. Two of those would be mirrored/duplexed to provide primary storage. The third would be used as a destination for an image of the working data set. The 160 MB/s transfer rate of Ultra160 SCSI theoretically allows you to transfer the entire 150 GB database to the target drive in about 32 minutes?with half the 160 MB/s throughput used for reads and half for writes?although in practice it might take twice that long. Still, in comparison to even a very fast tape drive, the disk-to-disk transfer takes no time at all. Also, that backup hard drive can be replicated offline on another system for redundancy. With a recent backup stored on a hard drive, if you have a catastrophic failure, you can simply insert the backup hard drive and run directly from it, without spending the time needed to recover from tape.
If you work with extremely sensitive data, using a removable hard drive allows you to secure that data by taking it with you or by storing it in a vault. When we first encountered a removable hard disk drive many years ago, this was the reason that drive was being used. The corporation ran two payroll systems, one for executives and the other for everyone else, and was paranoid that an employee would find out just how much more the executives were paid than everyone else. The executive payroll system used a removable hard drive, which was secured in the vault immediately after the executive payroll was run. And their fears were perhaps justified. If it's any indication, when we had to troubleshoot a problem with the payroll software on the executive payroll computer, it turned out that the problem was that the field for monthly salary could not exceed $99,999!