glyph external hard drive problem

I use iMac 10.7.5 and I recently bought a Glyph GPT50 ext hard drive for video storage. After a few days I noticed that the blue power-on light was off and the drive was failing to show up on my iMac volumes and the power wouldn't go back on. I disconnected, reconnected, etc., restarted, etc., but the drive still wouldn't work. I then replaced the firewire cable and plugged the drive into another power source, to no avail. I sent the drive back to Glyph and they returned it the next day and it worked and all of the files I had stored on it were intact. They said there was nothing wrong with it, that it was probably my power source. But I have other electronic devices plugged into the same source and they all work. In any event, a few hours after the just-returned drive was connected the light flickered, the power died and again it stopped showing up in my volumes. I sent it back a second time, they replaced it with all stored files intact and it worked fine - for 3 weeks. Today it was off again and not mounting. Does anyone have any idea why this is happening?