What you should do BEFORE you make any changes to the computer to test your theory is: Check and confirm that your client's files has been backup.
Troubleshooting is the process of detecting issue on a computer system so as to correct those issue detected.
Before you make any changes to the computer it is important that you verify in order to confirm that recent backup of your client's files has been created as this will help to prevent your client's from losing important files after the changes has be made on the computer.
Inconclusion what you should do BEFORE you make any changes to the computer to test your theory is: Check and confirm that your client's files has been backup.
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