I want to take my iMac to a technician who will put in a new 1 TB hard drive. Before I take it in, I will back it up to a Samsung external drive, but I want to TOTALLY protect what remains on the old drive, so that the tech cannot access ANYTHING. How can I do that?
iMac 21.5″, macOS 10.15
If you are very serious about preventing the tech from having access to your data, then this:
What to do before you sell, give away, or trade in your Mac - Apple Support
With this process you can wipe the drive and reinstall the macOS before you hand the machine over to your tech. He can replace your drive and clone the fresh OS to the new HD and hand the Mac back to you, after which you can recover your files from your Samsung backup.
I would advise you to make two backups with one being a Time Machine backup. You'll need a second ext drive for this.
If you are very serious about preventing the tech from having access to your data, then this:
What to do before you sell, give away, or trade in your Mac - Apple Support
With this process you can wipe the drive and reinstall the macOS before you hand the machine over to your tech. He can replace your drive and clone the fresh OS to the new HD and hand the Mac back to you, after which you can recover your files from your Samsung backup.
I would advise you to make two backups with one being a Time Machine backup. You'll need a second ext drive for this.
How to protect my IMac contents
