What external drives are compatible with My Cloud Ex2?

I purchased a Seagate 4TB to use as a external backup to the My Cloud Ex2. All attempts to use the drive as a backup failed. I spoke to WD support and they mentioned certain drives are supported for this purpose. I’ve searched the WD site for recommendations for just this purpose to no luck, is there a link to the suggested external disk that work with WD backups?

Thanks in advance

I was unable to locate a dedicated list of supported models, but most external hard drives should work as long as they do not use special drivers and receive enough energy (Devices with their own power adapters are recommended over devices that feed off USB ports).

Thanks for the input. NOT needing a drive with an external power supply would be the best, one has only so many outlets on a UPS.