You are probably trying to format the HDD while both it and the SSD are in the computer... Right or Wrong? If I'm right do as follows: If I'm wrong explain exactly how you have tried to format it.
To Format the HDD (only if the SSD boots)
1. remove the HDD and see if the SSD boots
2. If the SSD boots, remove the HDD and a) connect it via USB and format it... or b) remove or disconnect the SSD and with a Windows installer begin a Windows installation, format the HDD and stop there... Next connect the SSD and you'll have the HDD for storage. You can download the Windows ISO any version from a Microsoft download page and burn it to DVD or make a Windows USB installer. Or instead of using a Windows installer you can download a Hiren's Boot CD/USB ISO image and burn it to CD/DVD/USB.