I needed a way for testing buildroot kernel. For this, I'd put my build on a flashdrive.
I started by preparing a flashdrive (I happen to have a spare 2 GB one) with a new MBR partition table and an ext2 filesystem. Then I installed grub on it.
# mkfs -t ext2 /dev/sdb1
# mount /dev/sdb1 /mnt/sys...