External disk automatically reconnects after unmounting I have a weird problem on my 13" MBP (Early 2015) running MacOS 10.14.3 Mojave.

I have an external USB disk with a MBR partition scheme and one big exFAT partition that allows me to exchange files between MacOS and Windows.

Whenever I try to unmount / eject this disk, the single data volume gets unmounted, the disk gets powered down. After a fraction of a second, the disk however powers up again and starts spinning. So there's no way for me to cleanly eject this disk.

What can I try?
The problem no longer occurs once I upgraded to macOS Catalina.