Sounds good… 
Addionatly i have seen still strange percentage during copy progress.
http://forum.mikrotik.com/t/v7-5-stable-is-released/160518/92
Edit:
I can confirm. It is related to VETH.
http://forum.mikrotik.com/t/v7-6rc-is-released/161262/1
I deleted my VETH (RC2) and tried to repartioning. → No boot → Netinstall RC2
Fresh installation, partioning is OK!
Configured containers including VETH → No copy of partion possible
Deleted VETH → copy starts working again…
Good catch BMA ! Is it related to /dev in the filesystem ?? Strange