Expanding Storage

I currently own a 3TB My Cloud which I use to backup and share music and other files with other devices on my home network. I am now near the 90% capacity usage mark and so im looking to buy another hard drive to use through the USB port on the back of the My Cloud device. Is there certain hard drives I should look to buy or would one such as the wd my passport be fine for this? Im thinking more in terms of speed and reliability. I dont want to get an SSD as the price of those for a high capacity one would be massive compared to a normal hard drive.

Just about any USB hard drive (or flash drive) will work. The most important thing is to ensure it’s properly formatted in a format the My Cloud supports. See the following WD Support Knowledgebase article for more information.

Would a desktop hard drive be better suited to the task than a portable one? Since I wouldn’t need to move it around I thought a desktop one would be better/last longer than a portable one?

I use seagate desktop drives designed for 24/7 backup. It has been running for 5 years 24/7. Never touched it, extremely reliable.

Check it out (just for comparison).
Of course now they have more modern ones.