Today while installing another RedHat box via KickStart ended up with KickStart flaw which installed MBR onto my USB installation disk. While the rest of the data remained intact I did want to get rid of the unwanted passenger on my USB disk. Solution turned out to be simple:
# dd if=/dev/sdX of=/tmp/mbr count=1 bs=512
# file /tmp/mbr
/tmp/mbr: x86 boot sector; GRand Unified Bootloader, stage1 version 0x3, boot drive 0x81, 1st sector stage2 0x62c02, GRUB version 0.94; partition 1: ID=0x7, starthead 32, startsector 2048, 209715200 sectors; partition 2: ID=0x5, starthead 32, startsector 209717248, 767055920 sectors, extended partition table, code offset 0x48
From here on you can either save that copy (/tmp/mbr) in case things go wrong, and launch
# dd if=/dev/zero of=/dev/sdX bs=446 count=1
This will zero the MBR and get rid of grub (or other unwanted stuff you dropped there).
Note that we zero only 446 bytes and not whole 512 to preserve the partition table.
That’s it, done.