I just tried to add an extra CD/DVD drive and an extra Hard drive ... the orgional hard drive is in the "master" "plug" of the "strip type thing" and the other was on the "slave" one. I turned it on, all the plugs were in right, but the computer didnt get passed its first screen where a load of information is. I unplugged the "older" hardrive (it was from an older computer) and it worked. At the same time as i first added the extra hard drive i added the DVD rom and that works ... is that ment to come up in "My computer" (both of them) ...
Ive been told it could be the power problem, i need more power to support the extra stuff
First thing I'd suggest is checking to make sure that both the drives are jumpered properly. On the back of them, they should both be set according to their roles. The Master drive should have its jumper set to "Master", and the slave drive should be set to "Slave". The newer drives have another option that allows the PC to detect which drive is master and which is slave, by how they're connected on the IDE cable (is that right, adityamahesh?). I think that setting is "CM", but I don't remember positively...
If the "Master" and "Slave" jumpers are already set, you may need to hop on into your BIOS screen during system startup, and check some settings in there. I wouldn't recommend it if you're rather new to the whole thing. If you change the wrong setting in there, it might take awhile for you to figure out how to get it back to normal.