Although a hard drive is typically where an operating system is installed, there are a number of ways you can run a computer without one. Computers can be booted over a network, through a USB drive, or even off of a CD or DVD. When you attempt to run a computer without a hard drive, you will often be asked for a boot device.
Computer Startup
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When a computer first runs, a Power On Self Test (POST) is performed. This checks all of the hardware to make sure that everything is responding properly, then a series of system beeps are performed. The BIOS is loaded and the computer then looks for a boot device based upon the order of the devices in memory. These devices can be hard drives, network drives, external drives, or even removable media. Many servers are actual ran off of a single-network device with the hardware loading an operating system into RAM memory over the network.
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No Devices Found
If your system runs through the list of devices in the BIOS and none are available for booting, it will simply perform a series of PC speaker beeps and inform you that no bootable devices have been found. You will be presented with a blank screen, with a single message, and a blinking cursor. If there is an available device for booting, yet the machine is unable to recognize it, you may want to try modifying your BIOS settings and rearranging the boot order to recognize the device first.
Network Booting
It is increasingly common for a company to create discless systems that boot over the network. These systems are all controlled by a single server that simply virtualizes a desktop system on the remote machine. The advantage to network booting over local hard disk booting is that everything can be controlled on a single server, along with full redundant back-ups. This removes hard drive failure as a possible way to lose a full machine and is a great way to facilitate the use of cheap, easily replaceable hardware.
Peripheral Booting
Peripheral devices are commonly used to boot diagnostic tools and other Live systems, operating environments that are loaded entirely into the computer's RAM. Most modern machines can boot off of external hard drives, USB flash drives, and flash memory cards. In fact, many popular Cisco routers use flash memory cards as the boot medium or to extend storage for configuration files.