You may want to either move your post, or post in, the correct subforum for your device where users more familiar with it may be able to assist
OS5 My Cloud EX Series
https://community.wd.com/c/os5/my-cloud-ex-series/255
Per the product data sheet for the EX2 Ultra, it lists up to 20TB so likely one can use drives up to 20TB in capacity. See the My Cloud Hard Drive Compatibility Matrix (link below) which indicates drives up to 22TB for the EX2 Ultra.
Using the forum search (magnifying glass icon top right) may yield additional past discussions like the following:
https://community.wd.com/t/maximum-hard-drive-capacity-for-wd-my-cloud-ex2-ultra/216739/
WD has the following “recommended” hard drive knowledge base article listing WD drives up to 22TB for the EX2 Ultra:
My Cloud Hard Drive Compatibility Matrix
https://support-en.wd.com/app/answers/detailweb/a_id/32083/
