Hard Disk Speed is often measured by Transfer rate, operation rate under various conditions and Rotational speed is measured in RPM.
Transfer rate often considered to be the most important, is measured in MB/S. The burst rate is the interface speed, at which data already in the buffer can be transferred, but this has little effect in real world tests. The sequential rate, is the transfer of contiguous data from disk.
Rotational speed is measured in RPM (and higher RPM means lower rotational latency, as the required sector comes around faster).
Short for Revolutions Per Minute, RPM is used to help determine the access time on computer hard disk drives. RPM is a measurement of how many complete revolutions a computer's hard disk drive makes in a single minute. The higher the RPM, the faster the data will be accessed.
For example, if you were comparing two hard disk drives, one with 5400 RPM and another with 7200 RPM, the hard disk drive with a 7200 RPM will be capable of accessing data much faster than the 5400 RPM drive.
However, it is also important to note that a 5400 RPM drive will be much cheaper than a 7200 RPM drive.
While it may not be a noticeable speed difference when loading small files, the RPM of a hard disk drive can make a dramatic difference when loading large files or several hundred or thousand files. It is always recommended to get at least 7200 RPM drive.
RPM can also refer to the revolutions per minute a computer case fan or heatsink fan spins at. Generally, with higher RPM, the fan will move more air or remove more heat from the computer or processor. However, as you increase the RPM of a fan, you will also notice an increase in noise or decibel (dBa).
Short for RPM package Manager, RPM is an installation and packaging tool used with many different Linux variants to install Internet downloads. RPM uses the .RPM file extension when creating a package.
There is also another speed measure for HDD that is the operation rate under various conditions, this includes both seek and transfer. However, this is less important than the RPM.