After upgrading from DSM 5.2 to 6.1, NAS no longer starts
My hardware
HP Server Gen8 with 4 x 2TB HDD
My approach as described by Polanskiman.
Juns loader 1.02b modified accordingly (PID, VID, MAC-Adr. and Ser.Nr.. After inserting the SD card with the modified loader, the server and the boot loader started.
I saw the boot menu of the loader on the monitor and was also able to start the upgrade process from the PC with the Web Assistant. After that, the server reboots and since then the NAS is unreachable.