Don’t know if there is a verified limit to the size of the hard drive on can use on a single bay My Cloud. Haven’t seen anyone indicate the larger drives over 12TB will not work. I tested a 12TB drive in a first gen single bay My Cloud and it worked. See my post earlier in the current discussion where I show two 12TB drives, one internal on external on a first gen single bay My Cloud:
https://community.wd.com/t/guide-upgrade-a-working-my-cloud-to-a-bigger-hdd-easy-steps/243625/4
https://community.wd.com/t/guide-upgrade-a-working-my-cloud-to-a-bigger-hdd-easy-steps/243625/6
The only drive(s) I could not get to work in a first gen single bay My Cloud were SSD drives. Talk about that in this discussion:
https://community.wd.com/t/guide-how-to-use-a-2-5-inch-hard-drive-in-a-single-bay-my-cloud/256912
