Portable storage device
A portable storage device (PSD) is a small hard drive designed to hold any kind of digital data. This is slightly different from a portable media player, which stores and plays music and movies. 

Some are fixed size hard drives of 256GB, 320GB, etc.  Newer units are expandable using 2.5" laptop hard drives, allowing for an unlimited storage capacity, which is useful for video and images.

When travelling, a portable storage device may be a useful alternative to backing up or purging memory cards if a computer is unavailable for downloading.

Photography
One type of data that may be stored is digital photographs (RAW data), transferred from a digital camera.

Many PSDs will connect directly to a camera and copy the images, or they may provide a slot for a memory card to plug in, with or without a card reader device. Some early models allow the user to review the images on a colour screen, but at the top end of the market.

See also
 Computer storage
 Mass Storage Digital Class (MSDC)

References 

Computer storage media