Most, but not all, filesystems reside on the magnetic recording surface of a Hard Disk Drive, or HDD. HDDs are also called hard drives (because the disks are hard as opposed to floppy), disk drives, or sometimes just disks. In this document I will usually refer to them as disk drives or HDDs.
There are three basic types of disk drives that have been available over the years. These are IDE, SCSI and SATA. The primary differences between these types are with the drive’s electronic interface to the computer rather than with the disk itself. Although external USB disk drives are available, they are usually PATA or SATA devices in a simple enclosure with a conversion for connection to a USB interface.
The first hard disk drives available for personal computers was the IDE, which stands for Integrated Drive Electronics. These drives used a parallel ATA (PATA) interface rather than a serial interface for data and commands. PATA stands for Parallel Advanced Technology Attachment and was developed in 1986, about five years after the original IBM PC was introduced.
The original PATA devices had data transfer rates of 8MB/Sec. Later versions such as Ultra ATA had data transfer rates of 16 or 32MB/s. ATA-7 has a transfer rate of up to 133MB/s.
Only two devices, almost always disk drives or CD-ROM drives could be connected to a single IDE channel interface. As a result, many motherboards had two IDE channels so that up to four devices could be attached.
The term IDE is really a misnomer as it does not distinguish one drive type from another. All PC hard drives are IDE, Integrated Drive Electronics, regardless of their bus type, PATA, SCSI, or SATA.
The SCSI, or Small Computer System Interface, was developed in the early 1980’s and was used extensively for connecting hard drives as well as other devices. Originally supporting data transfer rates of 5MB/s, it is now capable of supporting data rates of up to 640MB/s.
Up to seven devices may be attached to a single SCSI channel, and the cable length can be significantly longer allowing for much more flexibility.
SCSI devices are more expensive than IDE/PATA or SATA devices
The SATA, or Serial ATA bus has become prevalent since the early 2000’s and it is now difficult to find any other type of disk drive. Almost all motherboards manufactured as of this writing have four or more SATA connectors and a few motherboards have no IDE/PATA connectors at all.
SATA speeds were originally about 150MB/s and with the advent of SATA-2, data transfer rates are 600MB/s. SATA-2 also provides better error checking and hot swap capabilities.
USB hard drives—those with real, physical rotating disks—are not really different drive types. They are generally PATA or SATA drives in an enclosure with a USB bus translator.
USB Thumb drives, which are basically RAM, are true USB devices.