Pamela wrote:
Marking a partition as active only serves to tell the BIOS that if it passes
control to that drive then control should pass direct to the boot loader in
the active partition.

That shouldn't affect a drive not specified in the boot sequence. Most of my
hard drives have an OS and an active partition (for hot recovery) but it
doesn't affect normal booting from another drive.

Quite often what happens is a drive which used to have an OS is reformatted
in a way that doesn't touch the boot sector. In a BIOS boot (ie not UEFI),
the process is that the BIOS looks for the boot sector (which is 512 bytes
only). If it finds something that looks like a boot sector it loads it into
RAM and jumps to it. The tiny amount of code there is supposed to chain a
secondary bootloader (like GRUB).

The problem comes if it can't find the secondary bootloader, perhaps if that
partition has been deleted. There's not enough in 512 bytes to do anything
about that, which means it just hangs at a black screen/flashing cursor.

If the BIOS is set to boot USB drives first, it might pick this up if
booting with a USB drive plugged in.

This seems possible although the earlier FAT32 formatting should have set
up the partition table correctly.

The boot sector can survive reformatting of partitions which causes the
above problem.

Theo