Just wondered what operating system you are using and what you are using the external drive for, is it data backup, ie just manually copying over data or are you running an automatic backup up program on it?
Close down the computer and switch it off. If the drive is still switched on, switch it off also. Wait at least 30 seconds, then switch on Drive only, wait again about 20 seconds then switch on computer and start up as normal. Wait until everything has completely settled down, and then see if you can access the drive in the normal way. If you can then I would advise you to delete the corrupt backup file.