If your computer doesn’t recognize hdd, start with the connection and power issues. In case of an external HDD, use other ports or cables; for internal HDDs, check their SATA and power connections. Then use Disk Management to check if your computer sees the drive. It may appear there but not in File Explorer; you just need to assign a drive letter. If your HDD is unallocated or RAW, don’t do any initialization or formatting operations on it if you have valuable data on it. Running tests on another computer will help to determine the reason behind the problem. If it is related to files’ inaccessibility, consider using Notchox as a recovery tool for Windows and macOS drives.