The installer lies.

When it asks you to confirm you're going to install to a device
it gives a big warning that installing FreeNAS to the disk
will destroy all partitions and data.

However the installer doesn't explicitly destroy any partitions, it
just splats an image with a partition table to the disk that was
selected.

However, if your disk is bigger than the install image, AND it had
a GPT partition on it in another life, then the backup GPT label will
survive the installation process.

This will cause problems when you go to boot FreeNAS, as the backup
label gets found by the FreeNAS kernel first, and the MBR label is
never loaded, so your root partition is "hidden" and you end
up at the mountroot prompt.

This could be improved, instead of just saying, hey we are going
to nuke everything, we could show any partition tables that are detected
and say, hey, are you really sure you want to nuke "this".

Ticket: #5087