Number 318 - November 2009

What Those Disk Drive and Thumb Drive Numbers Mean?
by Don VanSyckel, March 2008 WYSIWYG newsletter
If you read the ads, many prices continue to creep downward. Look at thumb drives for instance. One gigabyte thumb drives are common and 2, 4, and even 8 gigabyte drives are being advertised. It's to the point where you could have the OS (Windows, Linux, or whatever) on a thumb drive and keep your data on a pair of mirrored hard drives. Think about it, unplug the Windows OS thumb drive and plug in the Linux OS thumb drive. Then unplug that one and plug in the MAC OS thumb drive. Who needs dual boot when the whole disk fits on one thumb drive. Also one terabyte hard drives are popping up more often and I suppose they will be 'common' soon. One terabyte, wow. I can remember, barely though, when a 5 megabyte drive was a big deal.    What is a terabyte anyway? Digital memory and disk drives are accessed by addresses. Addresses are made up of a collection of bits. Most of us have heard of 8 bits, 16 bits, 32 bits, and possibly 64 bits. Each bit is basically represented by a wire that can have a digital one (1) or a digital zero (0) on it. Bits apply to both data bits and address bits but I'm only discussing address bits here. So, one bit can address two things, 21. Two bits can address four things, 22. As luck would have it, 10 bits, 210, address 1024 things which is close enough to 1000 for many people. So pretty soon we were calling 1024 bytes one K bytes just like a $1000 is a K.    As size grew memory got to 1024 K bytes and we needed another name, one megabyte. This is a break from money being a million. You never heard of anyone being called a megaire, did you? Next we got to	1024 M bytes and called this one gigabyte. In money this is a billion. As memory continued to explode we got to 1024 G bytes. This is one terabyte. In money terms this is a trillion. Not that you'd want to but the K, M, G, and T byte designators can be mixed. For instance a terabyte is a mega megabyte. Now a million of anything is a bunch and a million million is a whole lot of bunches. A practical example is how many pictures will fit on a one terabyte drive? Assume a picture is 4 megabytes:    Picture qty = 1 TB/4 MB = 1 M MB/4 MB = 1 M /4 = 1024 K /4 = 250 K pictures or approximately 250,000 pictures at 4 MB each.    One last thing to mention about disk size. All hard drive manufacturers, in their zest to have a better product, lie about hard drive size. Many manufacturers attempt to mitigate the lie by including a statement on the box that a MB = 1,000,000 bytes or some such nonsense and this is raw or before formatting. A MB is a K KB or 1024 X 1024. That extra '24' is actually meaningful; it's 2.4%. Then compound it:    1 MB = 1,048,576    1 GB = 1,073,741,824    1 TB = 1,099,511,627,776    So a TB is actually 10% larger than a trillion. If you buy a drive advertised to be a TB you'll get about a trillion bytes raw. Then format it an lose 10 % so you actually get about: 1,000,000,000,000 X 90% / 1024 /1024 / 1024 / 1024 or about 838 GB or 0.82 TB.
Number 318 - November 2009