How to set up back up to my "My Drive"

I believe My Cloud device supports automatic back up to an external USB drive. Assuming I have a USB drive connected, how do I set this up?

Are you talking about Safepoints, backing up a computer, or syncing files. Read the user manuals and I am sure it will help you out.

User manuals can be found here:
User Manual (PDF) (Firmware 04.xx.xx)
User Manual (PDF) (Firmware 02.xx.xx)

As SectorGZ pointed out read the User Manual, in particular Chapter 10. Note however that WD uses different terminology depending on which firmware version your My Cloud supports. For v4.x firmware the feature is called “Safepoint”, for v2.x it is called “Backup” (in spite of the error in the v2.x User Manual table of contents). And further while one can schedule Safepoints in the v4.x firmware, it appears one cannot schedule Backups in the v2.x firmware.

One further issue to keep in mind. One will need to use a USB hard drive that is at least as large as the contents to be backed up from the My Cloud to the USB drive.

Also note there are speed issues when copying from the My Cloud to an attached USB hard drive. One can search the forums to read about those other threads.