I recently bought a new harddrive and installed it and I backed up some of my data onto that drive. I want to clear off my older existing drive while mainting the data on my new drive...I've done this clearing before so thats not a problem but I'm worried that after I erase my old drive my computer won't detect my new drive and I will have to format that new drive to get it to work gain. I don't know if this is what will happen. Can you usually just swap around harddrives without formatting them with each swap?

As long as the OS and/or boot drive (C:)is not being swapped you should have no problem. An easy way to check prior to erasing is to disconnect the drive you will be erasing and see if everything else still works. If it does and data is accessible, you should be all set.

That's a very tricky proposition. You said "I backed up some of my data onto that drive." To make sure that the new drive is bootable and useable you need to copy the Operating System over. That inclides (at minimum) the c:\, c:\windows and c:\program files folders. The new drive's partition also has to be made active. The safest thing to do is use a program that is specially written to make an image.