The data can be distributed over the entire hard disk which means that it is not written sequentially. An index system allows the head arm to find all relevant data. The platter and the head arm are delicate, so they are protected by a steel case. This helps prevent damage to the hard drive under normal conditions. If the hard drive stops working or gets damaged, you could lose all of your valuable data. To avoid an emergency data loss, most people use a backup system that keeps separate copies of the original document files. Hard drive sizes are specified in terabytes (TB), gigabytes (GB), and megabytes (MB). Most Apple-made computers have a 500GB hard drive installed, and the latest models come in up to 1TB. Any hard drive can run on both Windows and Mac OS X platforms. Most hard drive manufacturers format their hard drives to work with every operating system on the planet. If you have a hard drive that is locked or specified only for a particular operating system, you can reformat it to run on any computer. Most laptop and desktop PCs use hard drives that range from 5,400 RPM to 7,200 RPM, while higher-speed hard drives can be found in high-end workstations and corporate servers. The disk access time is measured in milliseconds. Although the physical location of data can be identified with cylinder, track, and sector positions, it is actually mapped to a Logical Block Address (LBA) that works with the larger address range of hard drives. It is possible to connect your laptop hard drive to a PC. Hard drives remain a popular data storage option for consumers and businesses, despite the growing popularity and rapidly decreasing cost of nonvolatile flash memory in the form of solid state drives (SSDs). SSDs fit in the same external and internal drive bays as their HDD counterparts. While SSDs are much faster, more durable, and use less power than hard drives, they are also more expensive. SSDs are considered to be more suitable for applications that require high performance, while HDDs are more often used for high capacity use cases.