Have you directly attached the WD My Cloud to the Mac? If so, some reformatting may have happened. The file system oh drives may change if connected lets say from Windows to OS. My suggestion will be to set up the device on the Windows computer lets say and access it from both computers through the internet network but not direct connection.
Is there any important information on the drives? If not I would suggest you to reboot the device and format the drives. You can reboot when you go to Device Maintenance and then click on Reboot.
How to format: http://wdc.custhelp.com/app/answers/detail/a_id/11034
Let me know if that will help,