The My Book doesn’t backup the computer. You use software on the computer (Acronis True Image WD Edition for example) to backup the computer to the My Book that is attached to the computer.
When a USB hard drive is attached to the My Cloud’s USB port the USB drive will show up within the My Cloud as a Share that is accessible just like any other Share on the My Cloud. Any backup software that is capable of backing up to an NAS should be able to backup to a USB hard drive that is attached to the My Cloud’s USB port.
Read the My Cloud User Manual that ERmorel linked to above to learn about the My Cloud, its features and how to use a USB hard drive with the My Cloud.
The USB drive would have to be as large as the data you are seeking to backup from the My Cloud.
Typically the Safepoint (v4.x firmware first gen My Cloud) or Backup (v2.x firmware second gen My Cloud) programs that are triggered through the My Cloud Dashboard to backup the My Cloud to an external USB hard drive will create full sized backups that are not encrypted or compressed in any way.
