How does the format command prepare a disk for use?

Answer 1

Formatting a hard drive can/will do a few different things depending on the type of formatting you do, and what you are formatting. Floppy drives and hard drives are formatted differently.

Simple semi-technical answer

Formatting clears the FAT tables so the drive appears empty to anything using it. This way all files are effectively erased at once. Formatting may also actually clear every bit on the hard drive and re-write it with a zero. This takes much longer however.

Long non-technical Answer

For the sake of simplification, imagine your hard drive as a book. Initially, a book is just a bunch of pieces of blank paper glued together. Lets say it's 100 pages. You're going to use this book to make paintings on each page. You'll need page numbers and a table of contents so you can find each painting quickly. You also leave a little box on the bottom to title each painting, that way if you draw a duck, and the title says it's a bird, you'll know something is wrong, and disregard the painting.

So now you have page numbers, a table of contents, and a title. Those things all take up space. So you had 100 pages, now you only have 95 pages worth of free space.

A low level format will erase everything from every page of your book, so every page is as blank as the day it was made. It will then create an empty table of contents, make sure each page of the book isn't damaged, put page numbers down, and make a box on each page for the title. Usually, this level of housecleaning isn't needed. A high level format is much easier.

A high level format, or "quick format" as it's called in windows, will just erase the table of contents. All the paintings are still there, but as far as the table of contents shows, any page in the book can be used. Now you can write in the table of contents that page 12 has a bird, then you can turn to page 12, paint a bird over whatever painting was there before, and write "beautiful white bird, at sunset" in the title.

If you do a high level format, remember that your drawings aren't gone, they simply aren't labled. It is possible to take a high level formatted drive, and recreate the table of contents by looking at each page, and figuring out what's drawn. That is assumeing your drawing hasn't already been drawn over of course.

I hope my oversimplification helps. There are some terms that you may have heard:

FAT: File Allocation Table. This is the table of contents. There are two of these, they're identical so if one gets messed up, you can look at the other one to figure out what's missing.

Checksum or CRC: This is what I refer to as the "title" of each painting. Harddrives use these to make sure that the data written to the platter is read back correctly.

Sector: This is sort of like a page number. They are a predefined size, say 512 bytes. If you have a file that's only 100 bytes big, the harddrive will put that file into one sector, and the other 412 bytes will be empty. If you have a file that is 1000 bytes long, it will use two sectors (or pages) to store everything.

NTFS or FAT32: This refers to how the data is structured on the drive. You could very losely compre this to what language the description is written in. German, French, etc. === === Note that, while a low level format will make the disk unreadable to the average user, sophisticated and expensive techniques exist that can read even overwritten data on a magnetic medium disk. The DOD (US Department of Defense) disk blanking protocol required 6 passes of rewrite before they considered a disk cleaned. The first pass would typically be to set all the bits on -- so 111111. The second pass would be 000000. Third would be 010101. Fourth would be 101010. Fifth and six would be the same as passes one and two. As DOD requires this, we can safely assume that lesser methods of disk erasure are not secure. In the cases of really sensitive data, even massive degaussing is considered questionable. What works better is to remove the magnetic/ferric coating from the disk. Acids that etch metal and particularly iron work well for this, although the process is dangerous, and of course destroys the equipment.