Add another volume to your Mac’s internal SSD and try to eject it. All hell breaks loose as the Finder wants to eject the whole disk, it seems. Probably a 6-year old bug.
unmount
You’re in a rush to eject an external disk, but the Finder tells you it’s in use by unnamed apps. What should you do next, and what has Sloth to do with it?
When you try ejecting a single volume of a bootable volume group on an external disk, other volumes are also ejected even though you chose to keep them mounted.
Finder refuses to eject a volume because it’s in use, or Disk Utility complains that it can’t unmount a volume. Here’s what to do about it.
My two USB drives have started to eject themselves, suddenly and without any warning. Why?
The Eclectic Light Company 