I’m assuming you’re running Windows.
Sometimes my external drive fails to mount. What I do to force it is follow these steps:
Click start and open up a command line window. Either find it in the Accessories folder or type cmd at the search.
In the cmd window, type ‘diskpart’. A new command window might pop up. In the new window, or the same one if a new one doesn’t pop up, then type ‘list volume’. It will list a number of volumes on the system in the form:
Volume ## Ltr Label FS Type Size Status Info.
Find your hard drive in this list and note the volume number. I usually find the hard drive by comparing Sizes, but it should also show your hard drive’s name under Label.
Next type ‘select volume x’ where x is the volume number for your volume. So for instance if your hd happened to be listed as Volume 3, you would type ‘select volume 3’.
Now, type ‘assign letter=G’ or whatever letter you want your volume to be. Once you’ve done this, your volume should be mounted and you should be able to access your drive normally.