I'm not completely certain that I understood your situation. I wonder if it's possible to clarify the question? Otherwise Disk Management is a good place to start looking for a missing HDD.
If the HDD is brand new you might need to initialize it. This is needed if below the disk it says “Not Initialized”. Right-click on the Disk that says “Not Initialized” and click “Initialize Disk”. Then you will have the option to choose between MBR and GPT. If the drive is more than 2 TB you should use GPT, if not you could use MBR as well. Once the Disk is listed as Basic and online right click the unallocated space and click create a “New Simple Volume”. For the file system choose “NTFS” if you want to use the HDD on Windows.
Yep, the question is lacking enough detail to respond to properly. Sure we could guess, but its a pretty risky guess - well no risk for us, but for example if you do what Techpumpkin_WD says and you had data on those drives, now that data is 'gone' and to have a chance of getting it back you are going to have to go to other measures.
SNGX1275 Thank you for noticing. By tuyet nguyen if you have any information on this HDDs Initializing and/or formatting the drive will result in losing all the data stored there. The steps I have explained are applicable only for drives that are empty or if you don't mind losing the information stored on them.