I have an HDD My Book 500GB–got it 4 (or 5) years ago. It disconnects and then reconnects by itself. It disconnects itself once I enter one of the biggest folders (or randomly).
I’m really afraid because my HDD contains a lot of important stuff and I don’t have a back up yet.
Try to backup any important files that you might have saved. As a troubleshooting step you can try using another USB cable or connecting the My Book power cable directly to the wall. You can also run a test using DLG, check the link below for the steps.
How to test a drive for problems using Data Lifeguard Diagnostics for Windows