Don't expect this command to work well across networks. Nice if you want to get an overview without fear of pressing the wrong keys. Doesn't work in DOS 3.30. Writing the master boot record to the hard disk in this manner can make certain hard disks partitioned with SpeedStor unusable. It can also cause problems for some dual-boot programs, or for disks with more than 4 partitions.
Specific information is below.
At the end of the ROM BIOS bootstrap routine, the BIOS will read and execute the first physical sector of the first floppy or hard drive on the system. This first sector of the hard disk is called the master boot record, or sometimes the partition table or master boot block. At the beginning of this sector of the hard disk is a small program. At the end of this sector is where the partition information, or partition table, is stored. This message should be erased and the master boot code rewritten before Setup is completed.