2 possible causes

1)
The device is unformatted (or in a format that you can't read)

Plug the device in, and fire up a partition editor
See if you can actually see the device

2)
It's not a USB mass storage device
but instead uses a different protocol (or it's own custom protocol)

You can't view the contents of some cameras in a file manager, due to the camera not exposing the filesystem over USB
instead they use a protocol called PTP

If you could post the exact make, model of the device

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In any case,
could you plug the device in, wait a few seconds, and post the last ~20 lines of
Code:
dmesg