Harddisk image

Our harddisk images are the same as floppy disk image: it's just a huge file of bytes which represent the bytes on the disk. So, a harddisk image of 1024*1024*100 bytes has 1024*1024*100/512 sectors (204800). All partition info and the MBR is on those sectors... AFAIK this is also how blueMSX handles harddisk images.
This is compatible with how you have access to disks in Linux: if you make a disk image, you can just cat the device (e.g. hda) to a file and you get exactly such a disk image (AFAIK, at least for floppy disks, this is true, but I don't see why it would be different for harddisks).

Thinks all for your help. What i (think to) understand right now:

The first 512 bytes start with EB FE 90 MSX_IDE. This is the MBR. It also contains info on where the partitions are on the image. All partitions start with EB FE 90 IDE_PART. These are normal disk images like F8 or F9 or a harddisk image with F0. Am i right so far?

I figured out how to determine the start sector of a partition and its length. Both are 32 bit numbers. What i didn't understand was the meaning of the other bytes near these numbers. I'm looking for a description of these bytes. (i'm sure i can figure it out studying the openMSX source, but i'm feeling rather lazy at the moment...

Be careful to know that the fdisk packaged with the sunrise interface doesn't number the first 4 partitions as 'usual', if you fdisk on linux or windows, partitions and drives are the other way around (partition 1 becomes MSX drive D:, partition 2 becomes MSX drive C:, partition 3 becomes MSX drive B: and partition 4 becomes MSX drive A

Thanks for the info. Another question: should the partition which has the boot flag enabled always be assigned to drive A: ?