[CentOS] unable to stop drive timeout was reached

On a fully updated Centos 7 system with the Mate desktop, I've just encountered what appears to be a bug in.. something?

I connected a USB portable hard drive and copied some files onto it, then right-clicked the icon and "Safely remove drive".

After about thirty seconds I then unplugged the portable hard drive and removed it.

Several minutes later a window appeared on my desktop that said "unable to stop drive timeout was reached".

This is new; I've never seen it before so I think it's related to the last updates in some way.

Google finds several mentions of this error message, here is one example:

https://forum.manjaro.org/t/usb-drive-safely-remove-causes-delayed-error-message/47117

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