The act of formatting a memory stick removes all data being stored on the stick. If you just delete files from the memory stick yourself, some remnants of those files and other types of useless data will remain. Formatting the drive permanently erases all data from the drive and restores it to the way it was when you took it out of the packaging. If you want to use a memory stick to do something like boot your computer, you will need to fully format your memory stick before you can do that.
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The actual process of formatting a memory stick is quite simple. By double-clicking on the "My Computer" icon on your desktop you will bring up a list of all drives connected to the computer at that time. The memory stick in question will be listed among them. Simply right-click on the name of the memory stick and select the "Format" option to bring up the formatting utility. You will be presented with the option of doing a "quick format" where all computer files are deleted from the stick or a "full format" where every last bit of data is deleted, wiping the device completely clean.
A quick format will only take a few seconds to complete. A full format could take anywhere from a few seconds to a few minutes depending on the size of the drive, the amount of data being erased and the speed of your computer.
During the formatting process, users will also be presented with the option to change the file system that the memory stick is formatted in. A file system is simply a term describing the way the memory stick organizes your files. There are two main file systems: FAT32 and NTFS. FAT32 is mainly used on older computers due to its size limitations. The largest size a computer file can be in FAT32 is 4GB. NTFS is used in more modern computers and is currently the standard file system for all Windows-based computers. The maximum size a file can be in the NTFS file system is a whopping 16TiB.