In the end it was very easy to get my winchester drive to be bootable. I just had to read the manual really... This was what I used:
copy/boot dk0:rt11xm.sys dk0:
The trick is to already have a copy of the boot-file (RT11XM.SYS in my case) on the disk. The "copy/boot" command then just moves the file from one part of the disk, to the boot section. Easy.
I used that to make my Winchester Drive bootable, so now I'm booting straight from the fixed disk.