The key to the function of your computer's hard drive is called the index. This index stores all of the information necessary to find every piece of data on the drive when it is working properly. The benefit to you of the index is that it speeds up your computer exponentially when it has to look up something on the hard drive.
It goes to the index, which is really like the index in a reference book, to find the information that you or a program has requested. Once it finds the location in the index, it goes out to the area where the rest of the files are stored, finds the right file, and loads it into memory for use.
When information is erased from your computer, most of the time all that happens is that it is removed from the index. This makes the computer think it is gone. It may be weeks, months, or never before it is actually erased or overwritten. This is why computer geeks can often recover lost or deleted files from a hard drive. They simply scan the entire surface of the drive until they find any of the data that remains.
Unless you use a program that actually overwrites the file, it will remain for a while. Some data is left until the disk is either formatted or defragmented. Some of these files just sit there taking up space because they tell your computer not to delete them.
When you write files on a hard drive, they are allocated a little extra room beyond what they originally need. This way if you write to them again, the file can stay together. However, if you have added more files in the meantime, it may not have left enough surplus space. Your file is then broken into pieces and written in other blank areas of the drive.
This problem also happens when there is a space left from a deleted file. If the computer figures out that it is large enough to hold your new file, it will write it in that limited area. However, if you have deleted a lot of small files over time, your disk will have lots of little spaces that nothing else can fit into.
Now, you have two situations going. Files that are fragmented here and there on the disk, and little spaces on your drive that cannot be reused. This causes your computer to slow down because it has to read two, three, or more times to get one file. It also has to look at each of those small spaces each time it writes to check for reusable room. This makes your computer run slower because it is having to do a lot of additional work.
When you defragment a hard drive, it gathers up all of the pieces of each file and rewrites it in one piece somewhere else on the drive. As file after file is rewritten, the spaces that the file used to occupy are left blank. Eventually, all of those openings cause those little spaces to get linked together into larger and larger amounts of usable free space.
Ideally, when a defragmentation is complete, all of the files are in one piece, and all of the free space is one large block ready to be subdivided again. Your computer will usually recommend not to defragment a drive that is less than 5% to 10% fragmented. If you are defragmenting thinking it will erase all of your old data that you do not want recovered, this is probably not going to happen. Some will be written over, but not nearly as much as you think unless your hard drive is over 90% full.
