Mmm, I'm still having trouble visualising this. So you mean that the drive is a /dev but I don't ever see that in the file manager because it is formatted so the bits of data stored on it count as /mnt instead? I think my difficulty is because, even when it is a single partition, so it is just one "thing" and that thing is a "physical device" to my mind, I still see it as an /mnt not a /dev in Linux, whereas the description made me think it should show up as /dev.
Maybe the person who wrote the book threw me with the word "temporary" since that makes me think of a device like a USB stick plugged in for just a few mins or an hour - to my mind the data drive HDD inside the PC isn't temporary, it is permanent (well, as permanent as any hardware device is! They can all be removed or fail, so in a way everything is temporary).
So would this be a better summary (for myself)? >>>
/dev = physical devices attached to the system; some can contain data e.g. /dev/sda represents the first hard drive on your PC, but you won't see /dev/ as the path in File manager.
/mnt = a file system/partition stored on a /dev. Even though the files are on a /dev, the File manager will show the path as /mnt. This is normal, and you don't have to worry that seeing /mnt means you have set up Linux incorrectly - /mnt is the normal appearance for a browsable file system on a drive in Linux.
(The last bit means I haven't done it wrong and can tick this off!)