Check the filesystem and supported formats.
All my external drives were USB and I could just copy files onto them. I didn't use the software required as I saw no need to install it. Plugging in the drive enabled the computer to see it.
However, I wasn't able to back up virtual images as the files were too large (on one drive) and on another one I tried I found that several of the files I was copying over refused as the filenames weren't compatible.
Last time I upgraded the laptop (needed larger drive, and wanted a fresh OS install without risking existing data) I just bought a new SSD and an external drive caddy. New SSD went in laptop, new OS installed, old HDD went in caddy and data copied back.