I have been playing with Debian's Automated Install Using PreSeeding.
In ongoing operation, a device's bios is usually set to boot off a hard-drive as first priority, and via pxeboot as second priority. This means that when the hard-drive is virgin, or blank, we'll get a pxeboot. This is good for when first applying an operating system to a device.
But when it comes time to rebuild the device, or test different pxeboot provided configurations, changing the bios priority between pxeboot and harddrive can be cumbersome.
To simplify this situation, before rebooting a box with Linux installed on it, from root, the following can be run:
dd if=/dev/zero of=/dev/sda bs=512 count=1
This will assign zeros to the boot partition and over-write the master boot record with zeros. Upon reboot, since nothing can be read from the harddrive, the bios will initiate a boot via the network based pxe process.
WARNING: be sure of what you are doing. Fundamentally, this is a non-coverable operation and should not be performed on a production system.
To format the Master Boot Record (MBR) only:
dd if=/dev/zero of=/dev/sda bs=446 count=1
Source: Axllent.org: Erase your MBR on Linux http://www.axllent.org/docs/view/erase-your-mbr/