The 431933-B21 is an SAS hard disk drive by HP. HP makes hard drives for use in home PCs and for use in backup storage or at an enterprise level. As compared to simple SCSI or IDE hard disk drives , SAS hard drives such as the 431933-B21 have many superior features such as compatibility with both SATA and SCSI devices. This also means that (as of yet) the HP 431933-B21 and other SAS devices can have a maximum transfer rate of 3 Gb/s. This compared to SATA150 is a very real advantage but SCSI manages to meet this with Ultra320 (320 MB/s transfer rate). However SCSI has limitations in its development due to power consumption issues. The HP 431933-B21 has a 36 GB capacity and is ideal for use as database backup for websites or in other such situations where data might be required quickly. Unlike most hard disk drives the 431933-B21 has a form factor of 2.5”.