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#21
Check again scrolling down in Disk Mgmt to see if it is there but needing a drive letter assigned. Drive Letter - Add, Change, or Remove
BOot into BIOS setup to see if the HD is detected there. most often by Serial you can google to find make/model, and if not reseat it in its compartment to try to get it detected.
If it's detected in BIOS and still not seen by Windows then boot the maker's Hard Drive Diagnostic Procedure extended CD scan to see if it detects the HD