Final answer:
Testing the solution is the next step after replacing a damaged USB cable for an external hard drive to ensure the issue is resolved.
Step-by-step explanation:
After replacing a damaged USB cable that was preventing a user from accessing an external hard drive, the next action should be to test the solution. Testing ensures that the root cause of the problem has been addressed and the drive is now functioning properly. It is unnecessary to restart the computer or format the external hard drive at this stage, and since the issue was with the cable, trying a different USB port is not a relevant action.