Thanks Magic Banana.
Out of curiosity:
1. I've unplugged B (the important one) and kept A and C plugged (empty).
2. I've put C as ext4 and with a specific mount point. there's a file in it,
to see if it's still there after the install.
3. I configured A as LVM, and made / and /swap as logical volumes.
What scared me was the the message about formatting C.
I selected only A for LVM anyway, but still.
Anyway, I couldn't install GRUB for some reason (this machine is going to be
the end of me...).
Still, I finished the install without GRUB and started the live CD. And the
drive is not encrypted anymore, plus the file disappeared...
So for LVM, it seems to be much safer to unplug the drive with data, make the
install and configure the mount point in fstab.
Maybe the path was kept, since the drive is encrypted.
Not sure if info about mount point is also kept on the drive.
