I’ve had no problems using a number of different sized mechanical hard drives in a first gen. From 250gb up to a shucked WD Elements 12TB drive. See my posts in the following discussion:
I later put those shucked 12TB drives into a Synology NAS where they (knock on wood) are humming long just fine for the past year.
Only issue I’ve had is trying to use an SSD drive in my first gen. Tried several different SSD low sized (64GB and 120GB) drives, none would work for some reason. They failed to boot to the My Cloud OS. Cloned or using Fox’s directions didn’t matter when it came to using SSD drives in my first gen.
Currently have an old 750GB laptop hard drive in that first gen. 
