> I use KDE on Sid.
> Lately (but I can't pinpoint the exact moment), plugging an USB drive
> has stopped generating any reaction and I need to mount manually.
Are the udev events being generated? It looks like you can use udevadm monitor to watch what happens when you plug the usb drive in.
On my working system the udev events go to Udisk2, and kdelibs watches for for the events and uses configuration in system settings > device actions to decide what actions to make available.
I'm running a kde with a mix of testing/unstable and plugging a usb stick in working for me... even with udisks2 2.1.3-5