After successfully setting up a mirror of LogVol00 to a second hd, I've rebooted the machine without the second hard disk.
I was expecting a normal boot from the first hd, which was the one with the grub and the boot partition.
I've got a kernel panic instead.
To recover the system I had to boot in rescue mode
boot: linux rescue
The rescue mode won't find any linux partition even if asked to search for them.
So back to the shell:
The system complained about an unknown device within VolGroup00
lvm vgscanlvm lvscan
All logical volumes were marked as inactive
So, force the volume group activation
lvm vgchange -ay --partial VolGroup00
Now we can remove the missing mirror copy from the volume group
lvm vgreduce --removemissing --force VolGroup00
and the mirror logical volume will be converted back to linear.