Create raid gmirror after install
Is it possible to create raid 1 after install please ?
If yes, can you guide me how to do please ?
At the install, the firewall had 2 HDD, i did quick install.
No. Attempting to create a mirror after the installation will cause gmirror to overwrite some drive metadata which will break the filesystem. Current versions of FreeBSD won't even boot from such a broken mirror without setting a special sysctl flag.
Backup the config and reinstall using the gmirror option in the installer. Not only is that easier than doing the mirror after, it's safer and faster.
As a standard rule: pfSense stores everything in an easy to transport XML file and reconfigures all services to the settings in that file at startup. It is practically always easier to reinstall pfSense in the desired fashion and simply re-apply that XML file (either from the installer or from Backup/Restore in the WebUI) than trying to customise what's already there.