Is your store on the same drive as the OS? What other data do you have on that drive?
If you’re just wanting to move your Exchange store to a new drive, install the new drive, configure it to the system with a drive letter, open Exchange Manager, Under Servers->YourServerName->First Storage Group, right-click on Mail Box Store and select Dismount Store. Your users mailboxes will be unavailable during the move. Next, right-click on the Mail Box Store and select Properties. Select the Database Tab. At the top there are two fields showing the current location of your Exchange databases. Select the Browse button next to each of the fields, navagate to your new drive and “Open” your new Exchange databases on this drive. After you click OK Exchange will copy the database to this drive. Once you’ve finished don’t forget to re-mount the store. Then you’re done! Good Luck!