pk=Power Kernel. Not interesting in this context, but just mentioned to demonstrate a restore can be done while using another kernel version. Now what do you want to achieve? You currently have a proper functioning N900 as I understand (except from the USB) that you want to be able to restore without USB? Then start with installing the backup menu and make backups. It can always be usefull, if only to extract certain files from it. No guarantees everything will always work 100%, but a backup is a good starting point in case of trouble.