It has rebuild option in raid set up but if i apply this option it will erase all the data.
Unintuitive. If true, then the nvidia raid depends entirely on the software portion (ie OS driver) for the rebuild. And 'dmraid' in Linux assumes that the firmware does the work.
But "RAID is not a Backup". So you want a backup? Many options. Obviously, you cannot just copy files on live system, as files do change while you copy.
1. Go offline (boot to LiveCD/USB, etc) and 'dd if=/dev/source of=/dev/backup'. Bit hard remotely.
2. Build the system over LVM2. Make a LVM Snapshot, and then copy that. With sufficient space for snapshot, you should be able to finish copy in time.
3. Have the system on software raid (mdadm). 'mdadm' can remove and add disks to mirror and rebuild them online. Essentially same as your fakeraid solution, but we know the tools a bit better.