Does the computer still give you a "device connected" tone when you plug in the external drive?
If yes, it may just need a drive letter assigning to it, a common problem with external hard drives, especially if that hard drive is not frequently used with that computer.
Open Disk Management: Control Panel > Admin Tools > Computer Management > Disk Management.
Concentrate on the bottom-right panel showing ribbons which represent each detected drive.
Find the ribbon which relates to your Toshiba drive, it won't have a name and will have "Raw" as the description.
Right-click on that ribbon and choose "Change Drive Letter & Paths".
Accept the default letter or choose a different one from the drop-down list.
Click 'OK' and wait for the ribbon description to update itself.
Was this USB drive previously working in this PC .?
If yes try another USB port if front try rear ports
Try another PC
If no is this a 2.5" or 3.5" USB drive
3.5" drives need a psu
On some laptops there is not enough power (500mA) from some USB ports for 2.5" drives
Some USB cables have 2 connectors , one USB data + power the other one power only connect to two adjacent ports to get enough power