I had a problem with extending an EXT3 drive using LVM. I had two drives totalling a terrabyte and I was extending it with two more so that it would total 1.5 Terrabytes. The issue was when I went through the process clearly documented on all websites to do so, and then mounted the drive, the allocated space was reported as upwards of 50GB less than it should have been. What I didn't realise was that when you have a terrabyte and a half drive, you actually use up that much space on filesystem data. It's hard to believe that you can use up that much space on filesystem stuff, but then again this is a terrabyte and a half...