how to migrate RN2120 data to RN212

This is a community forum - you are not talking to Netgear support.  If you do want to reach support, then you can do that using my.netgear.com.  They will charge though (and you only have a question, not work for them to do).

Honestly, if the RN2120 is functioning (and actually running 6.0.8 firmware) you are better off copying off the data to backup, and just doing a fresh factory install on the RN212. 6.0.8 firmware was released back in 2013.  Current firmware is 6.10.4, and there have been a lot of changes since then.  If you are running with a 7-year old disk, I'd also suggest getting a new disk for the new NAS, and copying the data over the network. 

You will have a problem migrating a 6.0.8 disk to a new NAS - the RN212 system will attempt to upgrade the firmware on the disk, and that will almost certainly fail.  You need to upgrade your RN2120 firmware in steps - first to 6.2.5, then to 6.5.2.  After that you can safely upgrade it to current firmware, and migrate the disk to the RN212.  There have been changes to the file system since 6.0.8.  So post-migration it is best to copy off the data, do a factory default, and then reconfigure the NAS and restore the data from the backup. 

If the RN2120 isn't working, then you need to find a way to offload the data, zero the disk if you really need to re-use it, and then do a fresh install on the RN212.  Then you can restore the data after you set up the RN212.  That is the best path if the RN2120 is not longer working.  There is beta-level btrfs and mdadm software you could install on a windows system that might work.  Or you can use a linux system if you are comfortable with linux.  Both of those options are free.  You could also purchase data recovery software that supports BTRFS - for example ReclaiMe.  That is quite expensive though.