gmirror on a disk with data already on it

[Marc UBM Bocklet]

Hiho! :-)

The gmirror man page says that if gmirror is used on a disk, the last
sector of that disk is overwritten with the metadata gmirror uses.

So what I would like to know is, how risky is it to mirror a disk which
already has data on it?

Is there a chance that I might render my system unbootable?

Is there any way to find out what data that last sector holds?

Thanks in advance! :-)

Bye
Marc