After a month or so of running degraded RAID1 on the new disk to test it in our lab server (with almost daily backups on the old one) and since S.M.A.R.T. with its tests was silent, I've decided to finish the job of moving homes to the raid on the fly. The first step was done – repartitioning of the old one. Now I'm going to restart and move the old root to the end of the disk to have enough space for the raid partition. Keep one's finger crossed, please, I'll keep you informed.