What obvious trick am I missing?
Maybe I'm confusing it with the RL02 rails, but on at least one there's
a little metal tine on the top of the rail with a rubber "shoe". Lift
that up, and you can slide the drive forward about another half inch,
which gets it past the lock.
If the rack has been shipped with the drive in place but with no screws,
it's very likely that a piece of sheet metal is wedged under or into
the rubber shoe. Some judicious prying with a long screwdriver
usually straightens that out.
The tine/shoe keeps the drive interlocked to the rail until you lift
it and slide things about half an inch.
Tim.