When you have placed a new harddive into your external casing, it is normal that the harddrive is not yet recognized by Windows explorer
Windows works with logical volumes, that have to be created. The drive has to be partitioned and formatted.
When you have installed a drive that came from another system, like DVR, mediaplayer, NAS, Harddisk recorder, Gameconsole etc. it is possible that the drives won't be recognised by Windows. This is because it is formatted in a way that windows does not support. When you need to use these harddisks, you have to re-format the drives in the Windows NTFS file system. When this is not possible from the Windows disk manager, you could try 3th party apps, like Gparted and MiniTool Partition Wizard etc..
Please note ! : When you use an existing harddisk from another system and you repartition and format the drive, All data will be lost on that partition.
Please create a backup first, on the system where the harddisk was originally used. After this you can remove the disk and place it in the housing. We are not liable for any data loss.
Below you can find how to make a unknown partition available in Windows 10.
1. Press right mouse button on the Windows start menu icon and press on the diskmanager with the left mouse button. Disk Manager is started and you will see the screen below:
2. As default you will see that the new harddisk has the status ‘’Offline”. Please click right mouse button on "Offline" and select "Online" You will see that the drive is "Unallocated".
3. Press with "Right mouse button" on the "Unallocated" space. Now choose "Create new volume" , and select "New simple volume" and press Next.
4. The following screen will appear:
Windows will determine the maximum capacity of the volume. You could divide the volume into multiple parts, then you have to adjust the size of the volume. Press "Next"to continue.
5. You can choose a drive letter to add and press "Next" to continue. 
6. Now the hard disk has to be formatted. It is recommended to choose the NTFS filesystem. Volume name you can choose as you like. Press "Next" to continue. 
7. When done formatting, you'll see a summary screen, you can press "Next" to continue. Please wait a few moments to let Windows finish the settings. The drive will now be available in windows explorer.