Hard Disk Platter Presentation
When someone refers to a hard disk “Platter” they are referring to a CD like disk that holds all of the hard drives digital data. The hard disk platter is made of different materials depending on manufacturer but the most common materials used in platter construction are aluminum and glass. This is more of a ceramic glass not the type you see in a window. Older hard drives only had one platter, which made data recovery much simpler. Newer, high capacity, hard drives have multiple platters which makes data recovery much more difficult.
Hard drives today have multiple platters. In some drives as many as a dozen platters. Multiple platters make data recovery difficult, mainly, because of servo timing. Servo timing is like a map the drive stores in its firmware telling the drive exactly where all the data it has, on the platters, is stored. This information will be someone like file x is stored on cylinder 4, sector 1105, block 508. That means that the data is stored on the 4th cylinder and on that cylinders 1105 sector and that sectors 508th block. The servo timing lines up the bottom of one platter with the top on the next platter. If this relation between the platters shifts just a nano inch, then the drive will not know where any of its data is. This means the data will be lost forever.
Platters are manufactured using a process similar to CD’s called the splatter method. They are sprayed on and then coated with a protective film. Older hard drives has a layer of rust called “Thin Film Oxide” that also has a base lubricant in the mixture. The platters are then coated will a layer of magnetic coating, which allows data to be written to the platter. On top of that is a super thin layer that protects the platter from a head crash.
Another reason multiple platters make data recovery difficult is because multiple platters mean multiple read write heads. I will discuss read write heads in the following presentation but they are basically what read and writes data to the platters. The more heads the more difficult it is to replace the platters or the heads.
Many people think that data is written to platters in zeros and ones. This is incorrect. Data is written to the platters in a pattern very similar to a sound wave. In older hard drives data was written sideways along platter. This only allowed for a limited amount of capacity. Horizontal recording was used since hard drives were invented in 1956 till the year 2005 when the next generation recording was introduced, Perpendicular recording. Perpendicular recording writes data up and down on the platter instead of sideways. This is allows for much greater hard drive capacity. That is why today you see single hard drives approaching the 2-terabyte levels.
When a computer stores data on to a hard drive the data is encoded. This is done by a piece on the drive called the voice coil. The data is encoded so all the data stored on the hard drive is randomized. Data needs to be randomized so that the drive doesn’t get confused if two files look alike.
There are many interesting things about hard drive platters that many people outside of data recovery will never know. One of these is that platters are constantly losing bits while they are spinning. This means that during the normal operation of a hard drive, little pieces of the platter are being broke off and collected in little filter “pillows” inside the hard drive casing. Also, if the platter is scratched just a little bit data is destroyed forever.
Another thing many people don’t know is that platters, in their normal operation, have errors and bad sectors all over them. Even at the factory when they make a new platter and place it inside of a hard drive it is having errors. The first thing a hard drive manufacturer will do once a new hard drive is assembled is to scan it for bad sectors and errors. They will then hide all of these bad sectors in a file stored on the platters called the P-list or primary defect list. From then on the drive will hide bad sectors on its own in a file called the G-List or grown defects list. That is why when you buy a brand-new 250 GB hard drive it will only have around 220 GB of usable space.
When you go buy a new hard drive there are many different capacities to choose from. What many people don’t know is that a 500 GB drive and a 320 GB are exactly the same inside and have the same number of platters. This is because during manufacturing some sides of platters are damaged or defective. The other side of the platter is perfect, so the manufacturer sets up the drive to only read and write information to the good side and ignore the bad side. This means a drive may have three platters with all sides turned on to be a 500 gigabyte drive and another drive may have the same amount of platters but only four of the six sides turned on, to be a 320 GB drive. They save a lot of money doing this because if they didn’t they would have to throw away every platter that has a defective side. This doesn’t apply to just the 500 GB and 320 GB capacity drives but it may apply to every model drive that a manufacturer produces.
I have tried to include all the important information regarding the hard disk platters. There is some more information but it is way too complex and isn’t important to know about when dealing with data recovery. If you ever have any further questions about platters or anything about hard drives in general please contact Frank at Fhernandez@netcba.com or call (305) 228-6163.