Upgrading the BIOS may also require upgrades to the system board. You should contact DT Research here:http://dtresearch.com/support/support_contact.htm
They can tell you if this is something you can do yourself or if you need to send the tablet to them. I'm not sure where you are located so you may have a service center closer to you - they can tell you the closest location.
Not all tablets can have the hard drive added to them if it's locked out in the BIOS features. Maybe the feature needs to be turned back on - I have never seen one with this feature turned off, but it's possible. When the tablets are re-branded the third-party integrator decides what features to enable/disable. This is all part of upgrading the BIOS. DTResearch can usually tell you what needs to be done from the serial number so you can decide if it's worth the cost before you send it to them.
You can also sell yours on EBAY and buy one there that has the hard drive in it. WEBDT 310s with hard drives are common on EBAY lately. There is a person selling on EBAY often that use to work at a repair center and he sells modified tablets that he has turned on all features.