Check whether your external hard drive is formatted with NTFS.
NTFS is a file system originally for Windows. Apple only has the read privilege to Microsoft NTFS. To write to NTFS on a Mac, you can use iBoysoft NTFS for Mac to mount in read and write mode.
If your drive is not NTFS, it probably has some errors that even First Aid can't fix. Your only choice is to reformat it. However, reformatting the drive will erase all contents on it. You can copy files from the drive to your Mac before formatting.
- Open Disk Utility.
- Select the external drive on the sidebar.
- Click Erase on the toolbar.
- Rename the drive, choose a file system, and click Erase.
If you use the drive on both Windows and macOS, choose exFAT or FAT.
