Set default disk to the device mounted to root

debi.sh

@@ -214,7 +214,7 @@ sudo_with_password=false
 timezone=UTC
 ntp=time.google.com
 disk_partitioning=true
-disk=
+disk=$(df / | grep -Eo '^/dev/[a-z]+')
 force_gpt=true
 efi=
 esp=106