The dd example from BSDfan666, will copy the first sector, which includes the primary MBR. But, as I mentioned above, it will not copy any extended partition tables.
BSDfan666's example of backing up the MBR with fdisk saves the primary and all extended partitions in text format. The restoration is done manually.
To back up primary and any extended MBRs, just back up the first track:
# dd if=/dev/rsd0c of=mbr.backup count=63
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