Cannot cancel Disk Utility First Aid in El Capitan

Updating to El Capitan, I find that the First Aid function of Disk Utility can no longer be canceled.

Currently, it's been running more than 3 hours while scanning an external USB drive, and I need to take the computer elsewhere.

There is no red button, no cancel button, and Quit Disk Utility, though available, does nothing.

There is no indication of how much "repairing" is left to do.

My three options seem to be to Force Quit on Disk Utility, pull out the USB plug, or pull the power plug from the external drive. All three of these options seem likely to cause possibly serious issues with the external drive -- exactly the thing "Disk Utility" is meant to help with, not cause.

Has anyone encountered a way to stop First Aid in El Capitan without damaging the disk being repaired?

Thanks in advance!