First, see the following WD Support Knowledgebase which lists supported drives (including several Seagate drives) the My Cloud EX2 supports.
https://support.wdc.com/product.aspx?ID=906&lang=en#WD_compatibility
Second, see the following discussion that is relevant:
https://community.wd.com/t/diskless-ex2-list-of-compatible-drives/200804/
Third, read the My Cloud EX2 User Manual (https://support.wdc.com/product.aspx?ID=906) if you haven’t already as it may have further information on what drives are supported.
Lastly, see the dedicated subforum for the EX2. This subforum (My Cloud) is generally for the single bay/single drive My Cloud models. You can use the forum search feature, magnifying glass icon upper right, to find additional discussions about drive support on the EX2 model.
https://community.wd.com/c/network-attached-storage/wd-my-cloud-ex2
