I recently received a 4tb My Book. I replaced my 250gb MyBook. I also installed a 3tb cloud my book. I also have a 2 TB My Book installed. When the 4tb is plugged into the USB the computer freezes at start up. If I unplug the 4tb it will boot up. I can then plug it back in and it is recognized. I am running a computer that was upgraded to win 10 from win7. I had no problems with 2 My Books installed before this. I had the 250 GB and a 2 TB. Now with the 2tb and the 4tb My Books installed it will not boot.
Sounds like the PC is trying to boot from the external drive.
Check the settings on the Bios to see if the option to see if the booting sequence have changed and has the USB option enable.
I checked that and it wasn’t the issue. I had two externals on before, I switched out the 250GB for a 4TB drive. I’m wondering if it can’t recognize the larger drive. It’s an HP Pavilion. I didn’t see any spot where I could change the order anyway if I wanted too. The USB drives were at the bottom.