Generally yes, you can use any external USB hard drive or USB flash/pen drive with the My Cloud. In certain rare cases however some (including myself) have found that one particular external USB hard drive isn’t properly recognized and mounted by the My Cloud firmware. It is not confined to a specific brand, model or sized device. In my case I had to remove all partitions on the USB hard drive (a 1TB Toshiba Canovia hard drive) then create one single partition and format it before the My Cloud v4.x would properly mount the drive. See this discussion where I detail my issues and eventual solutions with that one drive.
The My Cloud User Manual indicates you can connect up to seven external USB hard drives/flash drives to the My Cloud using a (powered) USB hub. Most seem to have problems when attempting to connect more than four or five devices. I’ve posted elsewhere, see this discussion, of actually having seven devices connected when experimenting with connecting extra devices to a My Cloud.
Each external USB device that is connected will show up as it’s own Share in the My Cloud Dashboard. If one accesses the My Cloud Dashboard > Settings > Cloud Access section and sets USB Content Availability to On then the My Cloud Dashboard should aggregate (combine) the total storage space of all attached USB devices and show/report that information (including file information) in the Dashboard Capacity section. Note it is just showing the information not actually combining the external USB devices with the My Cloud, the USB device will still be treated as a separate Share.
Also note that there may be some speed issues when copying data to or from an attached USB device and with certain USB devices not entering sleep or hibernation mode. This has been well discussed elsewhere and you can find those discussions by doing a subforum search using the magnifying glass icon upper right.
