The most popular formats are UltraWide SCSI, running at 40 Mbps, and Ultra2LVD SCSI, running at 80 Mbps. There could be bays for 10 or more hard drives in a high-performance system. Some are hot-swappable cable drives and can be changed on the fly without shutting down the server. Most of the servers listed offer six or more hot-swap drive bays. Hot-swap SCSI drives usually come in 9G and 18G capacities, with 32G units now entering the market.
Some servers offer a reliability feature in the form of data mirroring.
When data is recorded to one disk, it is mirrored on a second. The best mirroring is provided via a hardware controller, but some operating systems, such as Microsoft BackOffice Small Business Server, Microsoft Windows NT Server and Novell NetWare provide software-based mirroring. This storage technology is also a cost-effective and efficient way to provide data security, and most server manufacturers offer it as an option. RAID combines two or more hard drives into one logical drive or array so that data may be saved on all of them. Advanced midrange and enterprise servers usually come with hardware RAID controllers.