HDD (Hard Disk Drive) is a traditional hard disk that stores data on rotating disks using a magnetic record. It is a “classical hard disk” - the most widely used type of disk on computers and servers from the 1960s to the present.
In 2008, especially on laptops, HDD has begun to get replaced by solid SSD drives that are much faster, but their lifespan and capacity are lower. Classical hard drives are still the best choice in price / storage ratio and so far, they are irreplaceable in terms of reliability and capacity. Although since 2017, SSD has been prevailing on laptops (notebooks), 2 in 1 devices and others, HDD is still being used in data centers (in clouds).
What types of hard drives are there?
HDDs come in two basic physical sizes:
- 3.5” is the most common and standard hard disk format used on most classic desktop computers and servers and it still has the best price / storage ratio
- 2.5” is a smaller hard disk format, which is mainly used in notebooks or in portable external storage. The advantage is smaller dimensions and USB powering. The disadvantage is lower capacity and worse price / storage ratio.
To select or to connect a hard drive, it is necessary to select the correct interface, that is, how the disk will be connected. The most common interfaces on computers are:
- SATA III (SATA 3, SATA 6G) - speed of up to 6Gbps
- SATA II (SATA 2, SATA 3G) - speed up to 3Gb / s
- PCI Express
- SAS - 6Gbps or 12Gbps
- M.2 SATA
- M.2 NVMe
- IDE (PATA) - not supported by manufacturers any longer, the use is declining
Most often, external drives are connected using the following interfaces:
- USB 2.0
- USB 3.0
- USB 3.1
How do hard drives work technically?
Every hard disk has several glass platters (disks) covered with a thin magnetic layer, that rotates at a certain speed (typically 5400 rpm or 7200 rpm). Above each platter there is a read and write head that does not directly touch the platter - writing or reading is done only by magnetizing a digital track on the platters. The number of hard disk reads and writes is almost unlimited in normal use. The surface of platters is divided into tracks, sectors and cylinders that form concentric circles - as the disk rotates.
The magnetic field also creates a footprint so you can trace the previous entries. This means that if you really want to delete your data from the hard drive so that no one can access it any more, it is not enough to just delete or format the disk, instead, you will need to overwrite the disk even several times so that all traces disappear completely. The second way of making sure that nobody can access your data after the data deletion is a disk encryption. This way, even if a potential thief manages to recover your disk, he will only get the useless encrypted data.