I'm thinking of moving to ARC loader of my old j3455 (before it was j1900) DSM 7.2-64570 installation - which was before dsm 6.0 surely - maybe even dsm 5 - I'm wondering how to check this btw - the initial dsm version.
In any case I found info in ARC wiki - https://github.com/AuxXxilium/AuxXxilium/wiki#important - that dsm v6 have different partitions layout, maybe this is related to your issue @Polanskiman
/dev/md0 of mine:
df -h /dev/md0
Filesystem Size Used Avail Use% Mounted on
/dev/md0 2.3G 1.4G 822M 63% /