I have a 4tb My Cloud which is nearly full. Before a I buy a second one I would like to know if I can have more than 1 unit on my system. Looking forward to hearing from you
I have a 4tb mycloud and a mycloud dl2100 on my system without any issues. So I think that you can have an extra cloud attacked too just give the new mycloud another name than the first one when you install it.
Yes you can use multiple My Cloud units on the same local network provided each My Cloud has a unique name.
If you do a forum search, using the magnifying glass icon upper right, you’ll find several discussions on using multiple My Clouds on the same local network.
You can also add an external USB3 hd to the USB port on the 4TB mycloud. I have added a wd5TB :WD Elements Desktop U3 WDBWLG0050HBK - Ekstern Harddisk - 5 TB that I use for a safepoint and give extra 1.5TB storing space. With a USB3 hub you can add up to 20tb extra space to the 4tb my cloud. You can expand by using 8tb units.
So I think you will hardly run out of space 
I do the same. Normally have a 5TB Seagate Backup Plus Hub attached to the USB port of the My Cloud to use as a Safepoint backup for the My Cloud and additional My Cloud storage. One can enable USB Content Availability under the My Cloud Dashboard > Settings > General > Cloud Settings to have the additional USB storage space reflected in the Capacity section of the Dashboard Home page.
There are some issues however with using the USB port. One is speed. Lots of complaints (do a forum search) about poor USB 3.0 speed when copying to or from a USB 3.00 hard drive attached to the My Cloud’s USB port. Then there is the issue of using multiple USB drives. While the My Cloud User Manual indicates one can use up to seven USB hard drives when one uses a powered USB hub, some of us have found that the My Cloud has trouble properly mounting and accessing more than four or five USB hard drives at one time. See the following discussion for more on using USB hubs:
http://community.wd.com/t/my-cloud-and-usb-hub-powered/168885
Many thanks
