Replying to Extra Hard Drive Advice

So i want to get an extra hard drive, just to up my memory space. Nothing too drastic, 100 or so GB would do the trick.

I would like to know from the techies here what brands are good, or what i should be looking for in specs to know it is good quality.

Also, once you have hooked up a new hard drive, will the computer flash warnings of low disk space if one of them is filled up, or will it recognise the added room. Also, will it then matter where things are saved for them to function, or is it the same?

Any knowledge on the matter would be greatly appreciated.