Western Digital WD20EARX 2 TB SATA 3 disk

Submitted by peter on Tue, 08/16/2016 - 11:32

Western Digital are one of the top three magnetic disk manufacturers with a market share slightly ahead of Seagate after the takeover of HGST, the old Hitachi disk business. Both WD and Seagate are double the size of Toshiba.

Samsung is another big disk manufacturer but is now focused only on solid state disks. Toshiba is moving toward selling only SSD. Western Digital moved into the SSD market by buying Sandisk.

The WD20EARS disk was introduced in 2010 and is labelled as “WD Green”. The various colours are connected to differences in price and warranty. WD Green gives you a slow rotation speed of 5900 rpm instead of 7200. The slightly lower power usage also produces slower seeks. If your computer is not battery powered, pay the little bit extra for full speed disks.

The WD20EARX-00PASB0 has a capacity of 2 TB, the largest in the WD 3.5 inch Green series way back when the series was created The disk has a buffer of 64 MB but that does nothing for the typical use of a low power “green” disk or a 2 TB capacity disk. The cache should be two or four times that size.

After you use up all the capacity of the cache buffer, the disk reads at a continuous speed of 35 MB/second, about the top end of USB 2 speed assuming your disk is in a USB enclosure and the only device on the USB path all the way through to the processor chipset. The continuous write speed is a little bit lower at 31 MB/second. If you put the disk in a USB enclosure, use USB 3.

The slow WD30EARS write speed creates a very slow backup. A 1 TB notebook disk would take 9 hours with large files and closer to 15 hours for many small files. You might run the backup overnight once per month then run something like rsync each day to backup only the changed files.

Your time is too valuable to put up with that speed. Plus a WD20EARS disk is five years old (in 2016) and at end of life. Buy a replacement. A modern 2 TB 2.5” disk will use less power, produce less noise, and work at least twice as fast. Modern 2.5” disks are so efficient that you can use them in simple USB 3 cases without an external power supply. I am replacing all my older 3.5” 1 TB and 2 TB backup disks with 2.5” equivalents.

I read old reviews of this disk. They are quoting speeds up to three times more than the maximum these disks can produce. How do the testers get such stupid measurements? They use tests that run mostly in the disk cache. Imagine buying a 2 TB disk then only ever using the 64 MB cache for storage. Stupid. Stupid. You buy a 2 TB disk because you need more than 1 TB. The benchmark should use more than 1 TB.

Advanced Format

The other problem, invented by Western Digital, is the very backward “Advanced Format” used on modern disks. Disks were set to jump from the obsolete 512 byte sector format to a more modern 4096 byte sector. Western Digital introduced the fake format with 4096 byte sectors on disk emulating 512 byte sectors and called the messy result “Advanced Format”. Now we are stuck with disks that have to jump through a slow process of reading before writing. Small random writes are very slow.

The controllers in early “advanced format” disks were bad at trying to improve the mess. Some later controller chips are better at aggregating IO operations into the 4K real blocks. If you have an early AF disk, replace it with a new AF disk.


While some disks are good for ten years, this model was obsolete almost as soon as I purchased one. There are several reasons to replace this disk and any one of those reasons justifies an upgrade.