Hello, glad to help you. According to your cases, there are two possible cases:
( 1 ) Please check the drive in the Disk Utility, if it is NTFS file system. This file system is not compatible with Mac, you may not see any of them in your folders but you can see on your PC. You can use the professional tool iBoysoft NTFS for Mac to mount your drive and you can read and write with it.
( 2 ) Check the drive in the Disk Utility, if it is not NTFS file system. It's mostly possible there is something wrong with your drive. You had better recover and backup all your data with some reliable software then fix it. iBoysoft Data Recovery for Mac is a good choice to recover all important in your drive.
- Recover all your data with iBoysoft Data Recovery for Mac first.
- Run First Aid in Disk Utility. Launchpad > Others > Disk Utility > Select your drive in the left sidebar > Click First Aid at the top menu > Click Run.
- Reformat the hard drive. Remember to recover all data with iBoysoft Data Recovery for Mac because this step will erase all data. Launchpad > Others > Disk Utility > Select your drive in the left sidebar > Click Erase at the top menu > Choose your wanted format > Click Erase.
If all of these won't work, maybe you can backup all data on you another computer and copy them with a new hard drive.