I have an external USB drive.
I have successfully installed 6.1 (2011-07-27) on the external USB drive.
And I it doesn't appear to have touched my internal drive (good!)

I previously had done this with 5.3 (?) and used
a mkinitrd command and associated cookbook found on a web page
(http://www.brunel.ac.uk/~eesridr/extlinux.html)

I believe that command puts certain things into a ramdisk, 
maybe for speedup?  I'm not sure what it does.

When I try that command under 6.1 I get a warning that it
won't override without --force.

I didn't do it.  Do I want to do it?  Things seem to
work ok, but I haven't done anything substantial with it yet.