Downloaded a "free" VPN software package. The package took over control of my MAC-mini, could not even reload the OS from the install disk! Decided that more space on a new disk was the way to go.
The replacement went smoothly. Just a little prying with a putty knife to get the case open and then eight screws to get the disk out. Excellent instructions ..
Bluetooth and WiFi antenna wires tend to get disconnected. Make a note of which one is connected where -- the connections on my cpu did not match the on-line instructions.
Always work on a clean level surface with a hard floor (no carpets).