Basically, the contents of your MyCloud are copied across to the backup location you specify. As noted above this can be a USB drive you attach to the MyCloud or some other location on the network. The next time you run the safe point process, it will look for changes between the files on the MyCloud and the backup, then it will update the backup accordingly. Anything deleted or added to the MyCloud will be reflected in the files contained in your backup. This is essentially a synchronising process rather than a true backup. Clearly you could delete an important file from your MyCloud then this would also be deleted from your safe point when you run the safepoint process. Hence it is better to run the safepoint say once per week rather than daily, but that depends on you. I have two MyClouds each with a USB drive connected directly to the MyCloud’s USB port. I find this works quite well for creating regular and automatic safe points.
Hopefully, it is clear from the above that your safepoint will take up exactly the same space on your backup drive as is on your MyCloud. Also, if your MyCloud fails you can easily gain access to your safepoint from another computer. This is easy if you use an attached USB drive. If you have critical data there is a lot to said for storing a backup offsite.