Gpart is a tool which tries to guess the primary partition table of a PC-type hard disk in case the primary partition table in sector 0 is damaged, incorrect or deleted. The guessed table can be written to a file or device.
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If you have booted with a rescue disk mount the floppy disk with gpart on, and cd to the mount point. Without any options, gpart performs a standard scan, and merely looks if it can guess a consistent primary partition table. Now if after the check-phase it says Ok, you should check the proposed partition table very carefully. When gpart has successfully written the new primary partition table, cross your fingers and reboot. If gpart says it found inconsistencies, you are a bit on your own.