First, check if you have a NTFS hard drive. You can do this by going to Disk Utility (by pressing Cmd + Space) If you have a NTFS file system, you can try the Paragon NTFS Driver (https://www.seagate.com/support/software/paragon). However, I haven't had much luck with the NTFS file system on Mac. So I've ended up formatting them to ExFAT. If you have an ExFAT hard drive that isn't mounting, try running the following commands - Plug in your hard drive Run ps aux | grep fsck - This will show if fsck is using your hard drive (typically happens when you don't eject it properly) Run sudo pkill -f fsck - This will kill the process and immediately show your hard drive in Finder Hope this works