Welcome to the TH community, @Corstiaan!
Can you please check if the HDD is recognized in
Disk Management in Windows, at least? Since the drive used be in a NAS system, you will need to re-format it first in order to be able to use it. The
re-formatting procedure will erase all the data on it, though, so make sure there's nothing important on that NAS HDD. Network-attached storage systems use a different file system for the HDDs, so they cannot be read from a Windows OS without third-party utilities.
Once the HDD is reformatted in NTFS (Windows OS-specific file system), you should be able to see, access and use the hard drive's capacity in the file explorer (This PC/My Computer) as you wish.
Keep me posted if you have more questions or issues with it!
SuperSoph_WD