RAM is temporary memory.
HDD space is for the most part permanent.
Defrag % has nothing to do with free space. To check your space right click your C:\, clic Properties and check the pie chart. It should also list free space.
If you have tons of old stuff you no longer want, go on a cleaning spree. Elsewise, get a new HDD. I'd suggest internal, but external is fine, I got a cheap 200GB HDD for 120bucks, now they cost a lot less, like 60~80 for 200GB? You can get internal 500GB HDD Seagate Barrucada 3.0GB/s transfer rate(Very speedy) for only 130bucks~
An important thing to know when you use 2 HDDs is that you should have all programs and your operating system on one disc (C:\), and all of your FILES on a seperate HDD (D:\, E:\, or w/e). It makes your computer run a lot faster ^ ^;
Gone with the wind
That's interesting...
Anyhow, yes you can have 2 at once. You can actually have more than 2 depending on how many ribbon cords you have inside of your computer, or rather ports to connect to them to. If you're not used to working inside your computer, go ahead and buy an external though. The good thing about externals is that it's a very simple swap when/if you get a new computer, or if you have multiple computers.
Gone with the wind
Hi, how to understand what takes up the most space on my computer. Here I open disk D, for example,but I can't really understand it.
Hi, there are quite a lot of programs for disk space analyzer. One of these is Dissy. You can find out at any time what takes up the most memory on your computer. For more information, you can check this out. I recommend it.