When the external USB drive is connected to the My Cloud USB port, access the My Cloud Dashboard and check the USB icon on the Dashboard: 
Check the My Cloud Dashboard > Share page to see if a Share with the name of the external USB hard drive is listed.
Typically if the My Cloud recognizes the drive it will indicate the status when that icon is selected.
If the drive isn’t listed or is listed but indicates 0K check the notification icon on the My Cloud Dashboard to see if there is an notification message that may indicate the problem: 
Also ensue the USB hard drive is in a format supported by the My Cloud: FAT32, NTFS (read and write capability), HFS+J, Linux Ext2, Ext3/Ext4, XFS
Unfortunately there are some external USB hard drives that are not properly recognized and or not properly mounted by the My Cloud. There are a number of other threads that mention this problem. For some (including myself) the solution was to remove all partitions on the external hard drive then reparation the drive with a single partition and then format it (in my case for NTFS).
Obviously this is not a good solution because you have existing data on your external hard drive. I had the same problem and ended up spending several hours moving the external hard drive contents to several other hard drives in order to clean it off so it could be repartitioned.
Here are some past threads, others can be found by using the magnifying glass search icon upper right:
https://community.wd.com/t/wd-my-cloud-wont-recognize-usb-drive/96017
https://community.wd.com/t/my-mycloud-doesnt-recognize-some-usb-hard-drives/152488
https://community.wd.com/t/usb-my-book-not-recognized-reporting-0k-available/96850
https://community.wd.com/t/wont-recognize-attached-usb-and-safepoint-no-longer-working/140464
