Block device names are unreliable on the long run, as they can change anytime you modify your storage connections (including through USB). As such, devices identified that way don’t reliably tell what they are.
Prefer using persistent names such as UUIDs.
Furthermore, if you are using Dolphin from within your installed system to spot that /dev/sdc is nowhere to see, this only confirms the behavior described above. If you had an issue accessing that drive, you wouldn’t be able to boot it to begin with.
On a side note, not seeing a directory is not the same as not accessing it: files and folders can be hidden, and access can be restricted even if visible.