Of course you can, why wouldn't you be able to? The D: drive is probably just an empty partition on your main hard disk.
Now, don't try to transfer programs from C: to D:, because you'll just break everything. If you want to run an application from the D: drive (which you can do, by the way), you need to uninstall it from the C: drive first, then reinstall it to D. You can also install new stuff you get to D:.
But if you're just in need of storage, you can move all your photos and music, or whatever you have, to the other drive.
In fact, it's probably a good idea to keep system and applications separate from your data anyway. having everything on the other drive also makes it easier to back things up.
You do have everything backed up, right?
· 1 decade ago