Could be several reason...
Is the HDD active.. Right click on the partitions and make sure they are Active.
Windows doesn't necessarily install all the needed drivers for your machine after a clean install, you may need to find them manually from the manufacturer or vendor website.
Make sure all cable and wires on the HDD are secure and not come loose of disconnected... it is possible.
Run the appropriate Diagnostic tool to see if there are any issues - see here -
http://www.briteccomputers.co.uk/forum/hard-drive-support/drive-diagnostic-utilities/Have you checked the device manager for any other issues?
Also have you turned on the windows update and installed all the important updates??You said you re installed windows 7 but had you windows 7 prior or another OS?