A half century ago floppy discs were still used as carrier medium. The PCs of this time dont even have a floppy diskdrive any longer, and so its not possible to read-out the floppys. floppy drive have a disk space about 1.44 MB. You could format them as required and to store new venues. Next was the CD-ROM.8 years ago, the notebooks had indeed already a classical CD-ROM drive, but it took some time to have the CD writer fitted as standard in standard PC. Until then, there were many computer programs to buy on CDs, but data storage was not possible on them. Quite at the same time then the USB sticks, SD cards and external hard drives came to the market place.
The data storage medium of today
The SD card has the benefit that it is smaller and is used as a storage medium in digital cameras, for example. As a disc away of such devices, it is rarely used, because not every computer has a SD card slot. Therefore these cameras always are supplied with cable to connect the device directly to the notebook. The external drive has large memory space, the current ones have 1000 GB, or 1 TB in size. However, most need an external own power source, they are big and unwieldy. Better as this is the Memory sticks. All laptops have a USB connection, the stick needs little power and is easy to take in your pocket. These 3 storage media are always refined and getting more and more storage space. The current speed of development is such that the capacity every year doubles. Still not in price. A year ago you got an external discdrive with 500 GB for about 100 . Now they are twice as large for the same cost.