I'm drilling right now.
Glass tank, diamond drill bit. Jumping on board this thread because I must be doing something wrong. I'm drilling 4 holes in my 120 (1/2" thick). I have been drilling for 5 days now (not continuous) and have only completed 2 & 3/4 holes. I have been drilling for three hours with a few breaks today and have only completed 3/4 of a hole.
Heres what I'm doing:
after marking where I wanted the holes I created a dam out of playdo and filled it with water to keep the bit cool. I didn't use a drill press or anything (which I'm sure is the way to go), just free hand. I'm drilling very slowly. Around 200 rpm per the instructions. Even slower in the begining to create a groove. After that I just sit there and let the drill do the work, without apply much pressure at all. Just sitting there holding a drill, trying to keep the trigger still and hold a even pace. Sitting there, until my arm starts shaking and I have to take a break, or the battery on my cordless drill begins to wear out.
Hopefully someone will post with some better input, but this is what Im doing and is how I understood it should be done.