Ubuntu Server 14.04. How to migrate the system to RAID 1?

Question

NAS

Hello. There is a self-assembly NAS based on - Intel 425KT, 2Gb RAM, 750Gb HDD, Ubuntu Server 14.04. There is no hardware RAID controller. I want to add a second HDD of the same or smaller capacity and make a mirror. There are two free HDDs for this case - 500Gb and 750Gb. It would be logical to use the second option, but the 750 is noisy, buzzing, but the 500 is quiet. In this regard, several questions.
1) How to migrate the system from one disk to RAID?
2) Is it possible to use the 500 paired with the 750?
3) And would it not be better to do not a mirror, but a backup of important data (which, for example, films and other music do not belong to) on the second screw? But in this case, the question arises of a quick recovery without physical access to the system. Indeed, in the case of mirroring, the system will still work even if one of the HDDs fails, and in case of recovery from a backup, you will need to do it yourself, which is not always possible, and on the NAS, in addition to transmission, minidlna and samba, yandex-disk is also spinning and ftp, which I actively use outside the home. In general, the dilemma is either a mirror, but with a noisy HDD, or a backup to a quiet HDD. Or setting up RAID on HDDs of different capacities.
I will be glad to any advice.

Answer 1

Convert the file system to LVM, then you can do whatever you want even on a running system.