Successfully updated my N54L to 6.1.4-15217 tonight following this guide. My first attempt failed after the first reboot after uploading the .PAT file. I chose 'migration' and I think this is what ballsed it up for me. I rolled back to 5.2 using the clean install option (the loader knew I was previously on 6.1.4 so migration was not an option) and to my relief my volumes were still intact. I then restored my config from backup and was back in business.
On my 2nd attempt I chose the clean install rather than a migration and this time it worked a treat and the NAS came back up and I was able to restore the config again.
If it is any help to N54l owners, you can see the built in NIC briefly turn off for maybe 2 secs during the boot stage (unlike XPEnoBoot you dont see the drivers load onscreen) and after another 20-30secs the NAS was pingable.
When it failed the first time, I didn't see the NIC turn off during boot.