Running rdisk /s does two things
1.)Updates the registry repair information which is kept under \winnt\repair folder. All the hives are kept there in compact format.
2.) Copies the repair folder to the floppy.
If the size is big, it fails to copy but whenever you really need to restore registry, you can just use expand command to extract \winnt\repair\<hive-name>_ to \winnt\system32\config folder. If NT is not booting, install a new one and from there restore it.
In fact, I have come across so many machines which don't recognise floppy drive till service packs are loaded, hence fail to repair using ERD. So, this is a workaround.