REMOVING A CRACKED GLASS BLOCK FOR REPLACEMENT.
Never done this before, so it's an experiment - try at your own risk!
(Use proper eye, face, ear, hand... protection. This creates thousands of sharp glass pieces.)
(1) I drilled the mortar around the cracked block I was removing with a 2 mm bit (1/8") at about 1 cm (half inch) intervals. First step in weakening the attachment to the surrounding blocks / wall.
(2) Then chiseled lightly between drilled holes to remove the cement. Glass expands when it breaks, so this step was to give the block expansion room so it would not expand and crack the surrounding blocks and tiles when I break it.
(3) Chiseled inward towards the center of the block I was removing, away from the things I wanted to protect. If the chisel wants to wedge or stick in cracks you are making, you're driving it in too far and the glass blocks will likely break.
(4) Prepared the hole by removing old mortar to receive the new block.
(5) Swept up 4 times (and am still finding broken glass!)
Taping over the block would have helped contain the glass shards.
(Re-edit for Annotations)
WARNING: Always use proper safety protection and proceedures. If you don't know get help or training or don't to it.