how to convert a non-RAID system to gmirror?

[Matthias F. Brandstetter]

Hi all,

I have two disk ad0 and ad2. FreeBSD 5.3 is installed on ad0, I can boot 
from it. Now I want to create a software RAID via gmirror with ad2.

My question is: how can I create the array with the spare disk so that I 
can reboot from it without loosing any data or reinstall the whole system?

Hope you can help me on this...
Greetings and TIA, Matthias

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