If you are in the field, have filled your memory cards with wonderful
music and want to record some more, what can you do? The digital
photography world has faced this problem and responded with portable
storage devices popularly known as "Digital Wallets" that can be used to
offload a CF or SD memory card so that you can use it again without
having to go home to your PC.
You should choose a model that has either a USB 2.0 and/or an
IEE-1394/FireWire interface to quickly upload the sound files to your PC
or Mac; USB 1.1 is too slow.
Updated 6/9/2004: The Apple iPod now has an accessory from Belkin called the Media Reader. It allows you to use the iPod as a digital wallet.
Updated 5/4/2004: Here's one that can store as much as 80 GB, has a rechargeable battery, USB 2.0 interface, and is reasonably priced (under $99 without a drive, eBay): LeadingSpects's Super DigiBin 2