I've bought an external DVD reader (Samsung, USB Slim Portable Optical Drive). But, it couldn't read CDs/DVDs on my PC. When I tried in another PC, it's okay.
What's wrong with my PC? The OS is Windows XP SP2.
Did you plug the reader into the front ports on your PC and does the reader have two USB connectors on the end of its cable? If so, try two ports on the back of the machine and see if it then works properly.
The issue is that many external optical readers/writers need to be plugged into two USB ports to help them get all the power they need to work properly - in other words, they need two USB ports'-worth of power, but due to the way some front panel USB connectors are wired up, they share a USB power feed so even using the two connectors on the drive's lead only gives you one lot of power.
Of course, this may not be the problem but I have seen exactly this situation on several brands of machine in our offices - the drives seem to be OK, but they won't recognise any media.
Executive Summary: Some external USB optical and disk drives don't work properly on a PC's front panel USB ports, but they will work fine when connected to ports on the back.