To empty the waste chamber, you must have both the priming port and SpotON port closed.

You can then withdraw any volume via the waste ports located on the left-hand side of the flow cell (marked in red):

Either can be used to empty the waste chamber using a P1000 pipette. As these ports are not closed, if the waste chamber is not emptied it is possible that some liquid could come out of this, which should not interfere with the rest of the system and can be wiped out.