Make two hard drives act as one in Windows 7

Can't afford a 1TB drive, but you've got two 500GB drives just lying around? Windows can combine them.
The Windows XP/Vista/7 feature is called spanning, and it can extend the usefulness of older, smaller hard disks. Here's how to create a spanned volume:
Step 1: Go to Start, then type "diskmgmt.msc" in the search box. This will launch the Windows disk management tool. Make sure that the disks you want to combine, are unformatted and don't contain any partitions. In our example, we're going to combine Disk 1 (298.09GB) and Disk 3 (186.31GB).

(Credit: Screenshot by Ed Rhee)

Step 2: Right-click on one of the drives to be combined, then select New Spanned Volume.

(Credit: Screenshot by Ed Rhee)

Step 3: When the New Spanned Volume Wizard appears, click Next, then add the disk(s) you want to combine, to the disk you already selected.

 

Step 4: Complete the rest of the wizard, which involves selecting a drive letter and formatting the new volume.
Step 5: Once you complete the wizard, check the Windows disk management tool for the volume you just created. You should see the new volume listed at the top. The disks should also be labeled in purple (Disk 1 and Disk 3 in our example).

 

That's it. We recommend including the spanned volume to your regular backups, because if one of the drives in the volume fails, the entire volume will be destroyed.

Actual Link: http://howto.cnet.com/8301-11310_39-57378002-285/how-to-turn-old-hard-drives-into-one-large-drive-in-windows/