Macbook does not recognize external hard disk

Hi everyone. I have a 12" Macbook running OS X 10.11.6. I also have a 1TB Western Digital hard disk, formatted as HFS+. I was using it as a Time Machine.

For some reason I cannot see the hard disk. Not in Finder, nor Disk Utility, nor Time Machine. Disk Utility actually gets stuck on ‘loading disks’ when the hard drive is plugged in.

But in System Information the Macbook does recognize that there is a disk plugged in. It is also listed when I do diskutil list.

I have tried rebooting, booting into safe mode (Disk Utility got stuck there as well), and resetting SMC as well as NVRAM. None of those things worked.

The disk is also not recognized by another (15" Macbook Pro) laptop.

What can I do about this?

Thanks!