While I was building an installer for debian on a usb stick, I ran the following:
install-mbr /dev/sdb
To fix a problem I was having at boot. It worked, but I then presented me with the following after booting the usb key:
MBR FA:
Some googling told me that I need to press “a” and then “1” at the next prompt to get this to boot, but reading the man page for install-mbr told me why I needed to do it in the first place (or at least a reasonable explanation for what happened).
It turns out the install-mbr command sets up a boot loader that will give you a menu if a key is pressed during startup. My best guess is that it reads the boot menu selection as a keypress and stops the boot at the mbr prompt. There are ways to disable this in the man page, but it isn’t worth the effort for me since I can just type a and 1 to proceed with the boot.