A lockout lever is hinged on the side of a slot in an I/O station of an automated library. The lever includes a leading blade and a trailing blade that protrude into the slot. The leading blade is urged open by a spring. When an LTO cartridge or a DLT cartridge in the wrong position is introduced into the slot, the forward motion of the cartridge is impeded by its substantially flat side that faces both the leading and trailing blades and prevents rotation of the lockout lever. When a DLT cartridge is introduced correctly, the cavity in its side accommodates the leading blade as the trailing blade is pushed out of the slot. The leading blade is sized to clear the gap in the door hinge of the DLT cartridge when removed from the interior of the station.