Usually, we are used to calling RAM primary storage. So the secondary storage means it is ROM. ROM usually refer to the hard disk in our computer. Being different with RAM, when our program was launched, this program is loaded into the RAM and when we shut down our computer, all data in the RAM will lose. The ROM is used to store the data of program, even we shut down our computer, the program data can be store into the hard disk, so if we want to continue our work, just reload the program into the RAM.

So the hard disk which is used to store program data is called secondary storage such as data traveler, mobile hard disk and so on. They are similar with the hard disk in our computer. When you want to use them, just plug them to USB ports of our computer through a USB wire. Then you can transfer data between your PC and data travelers.

Just be same with our hard disk, it is also divided by two kinds, HDD and SSD, usually SSD has longer life than HDD and it has better write and read speed, but its price is higher than HDD. So the data which is put in the SSD will be faster to be read and written. The data traveler and mobile hard disk support hot swap so you will not shut down your PC to plug the data traveler or mobile hard disk out. But things will be different if you remove you data traveler and mobile hard disk when they are writing or reading data. They are easy to get crashed!