Because your using an EX2 you may want to post your question to, or search in, the dedicated EX2 subforum where there are liable to be more knowledgeable people. Most threads and discussions in this subforum revolve around the single bay My Cloud.
https://community.wd.com/c/network-attached-storage/wd-my-cloud-ex2
There are several threads (more available via a EX2 subforum search) that may be relevant in the EX2 subforum. Just a few recent threads from that subforum:
https://community.wd.com/t/no-space-left-on-device-but-df-shows-1-22-tb-available/98614
https://community.wd.com/t/drive-not-full-but-cant-copy-more/98934
