Having problems with fixit floppy mounting filesystems

[Tom Parquette]

I was installing an additional hard disk and I believe I commented out 
the wrong line in fstab or typo'ed something.  The /usr filesystem is 
not mounted.  The machine will not boot to a command prompt so I can 
repair it.  It complains bitterly at boot time about not finding things 
in the /usr/...... directories.

I have never broken a system this badly before so this is the first time 
I have had to use a fixit floppy.
If I can get the root and usr filesystems mounted r/w I should be able 
to "ee" fstab and get myself going.

I can get to the fixit command prompt but when I issue mount /dev/ad0s1a 
/mnt to get the root filesystem mounted all I get is "operation not 
permitted".

I searched the mailing list archives and dug through the on-line 
handbook and I'm not finding anything that gives me a clue what I'm 
doing wrong.

Any help would be appreciated.
TIA.