Currently I have a 250GB drive and Writable CD/DVD. I wand to add a 3.51TB internal drive
you have such a small case, i doublt that there will be room inside the case for another internal drive. if there is room, the PSU should have an extra Sata power connector and you will need to get a Sata cable if you buy an OEM drive, if it's a 'retail' version of the drive then you should get a Molex to Sata power cable and a Sata data cable. you probably wont have space inside the case for another drive, go with a USB 2.0 external desktop drive.
You should be able to do so, provided you have the mounting space for the drive. You have to watch how you buy the drive - any OEM drive comes bare, with no cables (no SATA data cable). Other box stores selling drive may or may not include the data cable, you have to check. The power cable comes from your power supply, make sure you have a spare SATA power connector from the PSU...
PSUs with only one SATA power cnnector are quite rare right now; if your PC is up to date (say 3 years or newer) you should have two or more. If not, you can get a branching cable for about $5; that would require that the two drives are close to each other to connect both. You can get a simple converter for even less; that one uses one of the old style power connectors but gives you more freedom on where to put the new drive. You would need another SATA cable, too. Id go with a quality cable to ensure that it doesnt cause any issues with SATA II, which the new drive and your mobo probably support. Check if you want a straight cable or one that connects at an angle - they are both around $10 to $15. Hope that helps.
You will have to get a SATA data cable. The power cable going to your current HD has a connector for a second drive. _
