On occasion, I have found getting a hard drive in an enclosure to mount (or even be seen by the system) requires fiddling with the jumpers on the drive. Usually, they come from the factory set to "CS" (cable select) but, depending on the enclosure, you might have to set the jumper to "MA" (master).
Also, that mount(1) command you tried to issue, looks like it was probably pointing towards your boot drive (the one that's actually inside your computer).
If you want to get the device Linux is assigning to your external hard drive, run the following command, in a terminal:
Code:
$ watch -n0.5 'dmesg | tail'
and once your hard drive is turned on, plug it in to your computer. You should watch some stuff fly by, but what you are looking for is a line that looks something like
Code:
[179620.276803] sdc: sdc1
What the watch(1) command does, is it shows you the output from running the specified command (between the single quotes) at the specified time interval (in this case, every half-second). To get out of it, press Ctrl-C.