Strange, but I couldn’t empty the trash of a mounted SD card. Importing photos is no problem, because iPhoto offer to delete the photos after import. Importing videos with iMovie is different. When I copy the raw data only I have no option to empty the SD card.
Terminal to the rescue
chflags -R nouchg .Trashes/ rm -rf .Trashes/*
Caution: The first line clear recursively the user immutable flag