The platters are probably one of the most important components of a hard disk. The platter that is used in a modern hard disk is typically comprised of three different layers – the lubrication layer, the carbon layer and the magnetic storage layer. In the lubrication layer, a coating of Perfluoropolyether is used. The viscosity and inertness of this next-generation lubricant are both perfect for platter surfaces. The carbon layer (or diamond-like carbon layer) is used to prevent moisture seeping through to the magnetic layer. It is also nitrogenised to improve durability. The magnetic storage layer consists of cobalt, chromium and platinum. Cobalt is used to provide the orientation of the magnetic crystals, chromium enhances the signal-to-noise ratio, while platinum helps to stabilise the temperature. In order to reduce crosstalk between the layers, Ruthenium is also added to the mix.
Even before they leave the factory, disk platters already exhibit defects in the form of asperities. These microscopic “craters” of alumina on the platter surface are the by-product of the sputtering process (which is used to deposit metallic substrate on the bare metal platters). Even though these surface imperfections are minimised by burnishing and polishing, they cannot be totally eliminated from the finished product.
Manufacturers use what is known as a “P-List” to try and map out these defects so that the firmware knows not to write to these locations. (This list used to be on a piece of paper that accompanied a new disk, but was subsequently put on a 3.5” floppy disk.) Today, the P-list is stored on the disk itself. Run a “V40” terminal command on a brand new HDD and prepare to be shocked at the thousands of errors that have already been logged!) The use of “padding” around defective areas is another counterbalance employed to provide an extra safeguard against bad writes. With this technique, even healthy blocks around the defect areas are marked as “bad”.
Unfortunately, these seemingly innocuous blemishes on the platter surface can lead to potential data loss situations during the life cycle of the disk.
The above diagrams illustrate the life cycle of a platter asperity. Figure A depicts an AFM (atomic force microscopy) scan of platter asperities on a new disk. As disk usage increases (figure B), the sharpness of the asperity will erode a little. The prime cause of this wear-down effect is the sweeping effect of the disk-heads. In figure “C”, you can see disk debris now building up around the asperity. Eventually, the debris on the platters accumulates under the disk heads resulting in them retaining a higher voltage. This voltage increase leads to a higher “blocking temperature” in the disk-heads, resulting in them being unable to perform read operations on disk.
Last week, we were dealing with a WD Elements 2TB external USB disk experiencing the flaking platter problem. While it appeared to spin normally, the disk could not be seen by the customer’s computer.
After removing the plastic shell, we opened the internal disk (2.5” WD20NMVW with integrated USB connection) in our class-100 clean room and found significant amounts of platter debris which had accumulated under the disk-heads. We delicately cleaned the platters using a pharmaceutical-grade lint-free polyester swab and cleaned the disk-heads with same. The Head Disk Assembly (HDA) had to be removed for proper access to heads. Using the right cleaning methodology is key. Even using a “cleaning” chemical such as isopropyl alcohol or acetone can create smearing on the platters caused by the interactions these chemicals have with the air. Likewise, swabbing motion used must strike the right balance between debris particle removal and abrasion avoidance.
Once the cleaning process had finished. We put the HDA back in-situ. We reassembled the disk. We imaged it at a very low-speed onto a new USB disk. Once this process has completed, the disk was connected to one of our systems and the NTFS volume appeared in under 5 seconds! The customer could now be reunited with years worth of photos, Word, Excel, PDF and DWG files.
Drive Rescue are based in Dublin, Ireland and offer a complete data recovery service for 1TB, 2TB, 3TB, 4TB and 5TB WD Elements external hard drives. Common data loss situations we help with include WD Elements disks which are not being recognised by your computer or which are making a clicking, beeping or knocking noise. We can also help you if you can see your disk’s folders and files but cannot copy them. Popular models we recover from include the WD Elements WDBU6Y0020BBK, WDBUZG0010BBK-01, WD1000EB035-01, WDBUZG0010BBK-03, WDBUZG0010BBK-05, WD5000E035-00, WDBU6Y0020BBK-0B, WDBWLG0040HBK, WDBWLG0030HBK-04 and WDBU6Y0020BBK-05. You can find out more information our external hard disk recovery service here.