Clear MBR

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Sometimes you might end up with a corrupt MBR on your booting disk.

This may happen if you incorrectly install a new bootloader. Or any other reason.

Then it is useful to clear the MBR before rewriting it again.

We'll use /dev/ad0 as an example. You should ofcourse use the disc that is relevant for your setup.

You might have to set kern.geom.debugflags=16. Make sure you know what this means!

If you want to clear the whole MBR:

dd if=/dev/zero of=/dev/ad0 bs=512 count=1

If you want the partition table to remain untouched:

dd if=/dev/zero of=/dev/ad0 bs=446 count=1