MyCloud - retrieve data by inserting HD into other NAS with mirroring?

No. Other NAS manufacturers use different firmware and hard drive structures/layouts/formats.

If you want to retrieve your data you can remove the My Cloud hard drive from it’s enclosure and attach it to a computer running either Linux or a third party Linux driver. One can use a USB to SATA adapter, docking station or spare SATA port on their computer. One can use a Linux boot disk/USB flash drive or third party Linux drivers like that form Paragon Software (https://www.paragon-software.com/home/linuxfs-windows/).