First of all, you need to check the file system of the external hard drive used for Mac to see whether it is FAT32 or exFAT which is compatible with both macOS and Windows. If the problem doesn't lie with the file format when the external hard drive is inaccessible on Windows, you need to recover data from it and then fix it with effective solutions.
The file format of the external hard drive used for Mac could be APFS, Mac OS Extended, FAT32, and exFAT, only the last two are compatible with both macOS and Windows. In case it is an APFS/Mac OS Extended-formatted drive, you may not open the files on Windows PC.
If the file system is supported by Windows, be it exFAT or FAT32, you should be able to read and write the drive without any limitations. But if the external hard drive is not accessible on Windows, there could be some other errors on the drive or your Windows PC.
Whatever the reason, your data is at risk when the drive is inaccessible, you should recover data from it ASAP. As for data recovery software, I'd like to recommend iBoysoft Data Recovery for Windows. It can scan out the lost and deleted data on your external hard drive and save desired items to a different location.
To further fix inaccessible external hard drives on Windows, try these methods:
- Uninstall or update an old driver
- Change drive letter
- Take Ownership of the inaccessible external hard drive
- Add Authenticated User to the inaccessible USB drive
- Change the Advanced Sharing option
- Fix errors on the external hard drive with CHKDSK
- Send the external hard drive to the local data recovery company