installboot fails during install

[Riccardo Mottola <riccardo.mottola%libero.it@localhost>]

Hi,

I'm trying to install netbsd 7.0 from CD to a Thinkpad which currently 
has the windows + IBM recovery partition on it.
I proceed using the whole disk and installing MBR. Fine.
I get this error:

installboot -o console=pc,speed=9600 /dev/rwd0a /usr/mdec/bootxx_ffsv2
Old BPB too bug, use -f (may invalidate file system)


I have seen others have recently reported similar problems. The 
suggestion is to use "-f" which should work. But how ?

Apart the inconvenience that my favourite operating system fails to 
install straight away, when I go back to the install menu and I try to 
execute /bin/sh, I get a segmentation fault and get back to the 
beginning I tried several times.

If the installer can't make it alone (the best would be an option to 
"retry with -f at the user's will), I need a shell to execute the command.
How may I proceed?

Riccardo