* The partition with the ISO would be hd0s6 (I did dd if=debian-hurd.iso
of=/dev/sda6 on Linux), but if I try to write /dev/hd0s6 directly it fails,
and trying to mount the partition manually fails with a «Input Output
Error».
Ah! I hadn't understood that you were doing that.
You cannot do such thing with logical partitions: the dd call will wipe
the first sector which contains the logical partition chain cell. You
have to use a primary partition to be able to wipe out the first sector.
Alternately, since isofs doesn't actually use the first sectors, you can
skip one and seek one, to avoid wiping the logical partition chain cell.
* Installer asks for drivers, choose to manually select the drive.
* Then I change to TTY2, cd /dev and ./MAKEDEV hd0s6
This last step was not necessary in QEMU, that is why I was asking.
That shouldn't be necessary at all, /dev is getting filled with device
translator entries during the boot process, see attached snapshot.