Don’t know about the Gen 1 device but it is is almost identical to the mycloud EX2 (not ultra) and I have just successfully installed 2x10TB Red drives in one of these. WD, like most manufacturers, tests the systems up to a certain spec and won’t guarantee anything more, simply because they haven’t tested it. This is especially true of Legacy/EOL devices.
For the first-generation WD My Cloud Mirror (white body), Western Digital officially supports hard drives up to 8 TB per drive. That’s why the unit was originally sold with configurations that did not go beyond this capacity. The official compatibility list includes 2 TB, 3 TB, 4 TB, 6 TB, and 8 TB WD Red drives.
Drives larger than this, such as the 10 TB WD Red (WD100EFAX), are not officially supported. Some users have reported using larger drives successfully, but this depends on the firmware and how the drive is formatted. Since the device was designed when smaller drives were standard, larger disks may not always be detected correctly or could run into issues during formatting or initialization.
In short, 8 TB per drive is the officially supported limit. A 10 TB drive might work, but it isn’t guaranteed.