Backup by removing a drive

I have purchased a new hard drive for my cloud mirror, so I can remove one of my existing disks and store as a backup. Then I will rotate every few months or if I have a lot of changes. What would be the best procedure.
I want to do this because mycloud mirror was attacked by ransom ware and all files were encrypted. I have managed to retrieve the files but need to have a backup should it happen again.

If it is me, I would attach an external drive (8TB) to My cloud mirror and use the sync “backup” to copy data from the NAS to the external. If that just put the external drive away. The away you do, is the device will do a RAID rebuild every time you put in a different drive, if your device is setup as RAID 1.