Originally Posted by Kinetic
Assuming I know nothing about the /physical/ details of disk formats, how does such a system get going? Presumably the bootblock is readable by the code in ROM and it then contains enough custom load code necessary to read the next stage from somewhere else on the disk?
Yes, the bootblock is always the first place to check.
It either already contains the custom loader or it loads the loader/mainfile among other things. So you'd disassemble the bootblock and see what happens.
This is a very (very!
) simple explanation but I hope you get the point.