The problem with imaging one My Cloud drive to a hard drive that is a different size is the user partition may not be resized to take advantage of the larger space.
As Rac8006 indicates it is probably best to run one of the unbrick procedures then, using a SATA to USB adapter or a spare SATA adapter on a PC to copy the user data from the old drive to the new drive.
I posted about putting in an 8TB WD Red Drive a while back using Fox_Exe’s directions for a first gen My Cloud.
Note that the directions for unbricking a first gen are different than those for unbricking a second gen. User Fox_Exe’s unbrick directions:
First gen: https://drive.google.com/file/d/0B_6OlQ_H0PxVbzZac0plbkZKeGM/view
Firmware file used in first gen directions: https://drive.google.com/drive/folders/0B_6OlQ_H0PxVNTN6c2F4SXEwQVU
Second gen: https://drive.google.com/file/d/0B_6OlQ_H0PxVQVhnLVJOdDZISUU/view
Check the P/N ending number on the bottom of the My Cloud enclosure.
P/N “-00” is first gen.
P/N “-10” is second gen.
