Archive to external drive

my Mycloud Mirror drives are almost full and I would like to archive/change them into external drives, because I don’t need the remote access for those files anymore.
Can you help me with how to do that? Can I put the drives out of the Mycloud-device and use them as external drive? Or do I need another device for that?
I would like to fill the MyCloud mirror with new drives. And store the old drives in my office for possible use in the future, like an external hard drive.
Thanks for the help!

hard disks do not last long on a shelf before bitrot takes its toll

usb sticks are NAND based and also suffer from bitrot

only durable archive is blu-ray which are limited in capacity