If you plan on using an internal PC disk, where will you put that disk?
Generally one can use an external USB hard drive and attach it to the single bay My Cloud USB port. Then within the My Cloud Dashboard one can use either Safepoint (first gen My Cloud) or Backup (second gen My Cloud) to backup the entire My Cloud contents to that external USB hard drive. See the My Cloud support section for more information on how to use Safepoint / Backup.
If one wants to backup a My Cloud to a internal desktop hard drive they can use any number of free and paid third party software to either sync or backup the My Cloud across the local network to the internal hard drive. Or backup/sync to a USB hard drive attached to a computer. For example I use Free File Sync (https://freefilesync.org/) to backup a My Cloud to a USB hard drive attached to a computer.