Ok, seems there has been some confusion. Your non working system is emulating a DS918, your old N54 can only run as a DS3615. If you put the disks from the non working system into the N54 you will need to create a new boot stick using jun’s 1.03b loader, there will be a message saying your disks have come from a DS918 & that you can migrate to DS3615. At the moment DSM 6.2.3 is is the latest version you can run ok.
All of your existing data should be ok however it always advisable to have backups of anything vital.