Question: Is there any way to configure Windows 7 to make two hard drives act as one (without using RAID), so hard drive 1 and hard drive 2 would share a single drive letter?
Can this be achieved natively in Windows, or do I need some external software?
Answer: By far the easiest solution that technically meets the requirement is to mount the second drive as a subdirectory of the first. I.e. the second drive could be C:Drive2 instead of D:. But they don’t act as one, then: you don’t pool free space.
If you do want to share space, you’re talking about RAID-0 (or JBOD, Just a Bunch of Disks). Windows can do this without extra hardware. You’ll use diskmgmt.msc (“disk management” from the Microsoft Management Console) for this task.
First, convert both disks to “dynamic” disks. Right-click the disk and choose “Convert to Dynamic Disk”.
Next, you can create a volume on one disk and then extend it to the second disk. There’s one restriction: you can’t do this with the Windows boot disk itself. (That makes sense: you’re not using hardware for this RAID solution but Windows, so Windows has to be loaded before this works)