Issue With Partitioned Drive

I'm trying windows 7.

After having installed it I've seen something strange.

I usually have the disk drive partitioned as following:

C partition, the root one (with programs and OS)

D partition, with all the stuff (projects, downloads, etc.)

Now, after the first boot in my computer I've seen only the partition C.

So, I've opened the computer management / Disk management. Here I've found the partition D.

So, I've assigned a letter to the partition D and now win7 seems to see that partition correctly.

But if I try to do a scandisk (via TOOLS / Error-Checking), a message appears asking to dismount the partition: 'Windows can't check the disk while it's in use. Do you want to dismount this volume first? Note: All opened handles to this volume will become invalid.'

That's sounds really weird to me. Now my doubt is how win7 is managing this partition.

Any hint?