Most new PCs today come with capacious hard drives, with more than enough storage space for your word processing, spreadsheet, and photography needs.

But auxiliary storage can come in handy. The same is true with digital devices you use with a PC, such as a digital camera.

The most successful auxiliary storage device is the USB flash drive. It's known by this name because these devices have a fast USB (Universal Serial Bus) interface and because they use flash memory, which is a hybrid between random-access memory (RAM) and hard drive storage. Like RAM, flash memory is lightning fast (the "flash" name was coined by Toshiba to connote speed). As with a hard drive, data is retained when power is no longer being supplied.

USB flash drives are typically in the shape of a fat stick of chewing gum; their small size makes them very portable. You can slip one into your pocket or pocketbook, attach one to a keychain, or fasten one to a cord and wear it around your neck as a geeky fashion statement.

These drives can take other forms as well, including a pen that writes, a credit card, a bottle opener, and even a piece of sushi. This makes them well-suited for marketing, with companies putting their logos on the outside and promotion materials on the inside.

USB flash drives are also convenient for moving a lot of data between your office computer and your home computer, backing up important files, and running programs on another PC.

In 2000, IBM sold the first commercial USB flash drive in the U.S. It had a capacity of 8 megabytes (8 million letters, numbers, or special characters), which was more than five times the capacity of the floppy drives that were still in wide use at the time. Today, USB flash drives typically start at more than a hundred times bigger, with sizes ranging from 1 to 16 gigabytes (1 to 16 billion bytes), and they typically cost from $10 to $150. SanDisk (www.sandisk.com) has a range of well-regarded USB flash drives, and other vendors include Kingston (www.kingston.com) and Verbatim (www.verbatim.com).

For larger storage requirements and even faster transfer speeds than USB flash drives, external hard drives are a good choice. External hard drives come in capacities today of up to 1 terabyte (1 trillion bytes), as large as internal hard drives. This makes them well-suited for backing up the entire contents of your hard drive for quick recovery in case of a hard drive failure.

External hard drives are portable, but because they're more fragile, larger, and heavier than USB flash drives, they're not as portable. The HP Pocket Media Drive (www.hp.com) is an innovative compromise. It comes in sizes up to 250 gigabytes and is smaller than a paperback book. It works best with HP computers that have a Pocket Media Drive bay-you simply slide it in and out-but it also works with any computer via its supplied USB cable.

The French company LaCie (www.lacie.com) offers a similar product, called Little Disk, available in sizes up to 500 gigabytes. Like the HP Pocket Media Drive, the LaCie Little Drive is widely available at online stores as well as in some brick-and-mortar retail stores.

When it comes to digital cameras, manufacturers almost force you to buy auxiliary storage because the storage that comes with the camera holds a limited number of pictures. Unlike computers, a range of incompatible branded technologies and physical sizes are used, including but not limited to CompactFlash cards, Secure Digital cards, Secure Digital High Capacity cards, Memory Sticks, and Microdrives. You can check your camera's manual for compatible products.

Digital camera manufacturers sell auxiliary storage for their cameras, but you can often get better prices from third-party vendors. SanDisk has useful product compatibility information (www.sandisk.com/compatibility) that lets you see which cards work with a range of different devices.

Other digital devices, including cell phones, MP3 players, hand-held computers, GPS devices, camcorders, and game consoles, can also often benefit from auxiliary storage. As with digital cameras, it can make sense to check with both the manufacturer and third-party vendors.

Since at least the time when the ancient Sumerians pressed styli into clay and the ancient Egyptians marked papyrus with reeds, people have stored information. According to market research firm IDC, the amount of digitized information stored in 2007 reached 281 exabytes, or 281 quintillion bytes, due to the explosion of digital cameras, digital TVs, surveillance cameras, and social networks.