I would like to know if I was to swich from the WD my personal cloud device to the mirror version would i lose my files?
No, you will not loose yourf iles. You would simply copy the files from one device to the other. What you would also have to do is recreate the users/passwords on the new device if you want to keep the same user/password from the old device. There may be additional configureation needed on the My Cloud Mirror beyond the default/standard setup of the base WD My Cloud.
Your files will remain on the WD My Cloud untill you delete them (or format the unit).
How can I copy/transfer my files to my new 4TB My cloud, from the old 2TB HD ( now outside the enclosure), because I replace it. If I hook to a SATA port on my PC, can’t see the files or content ( any software to make fast the transfer).
If using Windows you will need a third party Linux driver since the My Cloud disk is in Linux format. The following thread discusses the issue:
Note: Post edited with correct link.