Generally (if I remember right), you cannot specify the files that Safepoint backs up. It is generally an all or nothing affair with Safepoint. Instead you will have to manually copy the files/folders to the USB flash drive either when it is connected to your computer or when connected to the USB port on the My Cloud.
There are ways to use SSH to automate a backup using rsync where one can select the files to backup (or to exclude while doing a backup). There are a number of past discussions that can be found using the forum search feature (magnifying glass icon upper right). Here is one example discussion:
Note: One proceeds at their own risk when using SSH and rsync.
If you are using Safepoint you need a storage device that is as large as the data you are backing up. If your 4TB My Cloud has only 1TB of space used then you’ll need a storage device large enough to hold that 1TB of data. If 3TB are being used then you need at least a 3TB device to store the data.
Generally external USB hard drives are cheaper (price per TB) the larger capacity one buy’s. One can find portable hard drives (2.5 inch type HDD) in the 4TB to 5TB range for $90 to $110. Sometimes cheaper when on sale. The larger external desktop hard drives (3.5 inch HDD) are typically cheaper, on a per TB basis, the larger capacity one goes. 4TB external desktop (3.5 inch) USB hard drives typically go for around $80 to $100.