In Finder, select the external drive and press Command + I. Check Format and Available. macOS can typically read NTFS drives, but its built-in tools do not write files to them. If you need to change the format, copy the files to a different drive and verify the backup before erasing anything.
01Check Format and Available in Finder
Section linkIf you can open files on the drive but cannot save to it, select the drive itself in Finder, then choose File → Get Info. Check Format and Available. If only a large copy fails, compare its size with the available space and test with a small disposable file before changing the drive.
02If it says NTFS, plan where to put your files
Section linkIf you only need files from the drive, copy them to the Mac or another drive with enough space. If you also need to write to the NTFS drive, you can use a Windows PC. For a drive you plan to reformat after backup, APFS suits current Macs and ExFAT works with both Mac and Windows. Erasing in Disk Utility removes the existing files. Open important files from the backup and double-check the drive name before erasing.
03If the format is already writable, check the volume’s purpose and permissions
Section linkIf this is the volume used for Time Machine backups, use a separate volume or drive for ordinary files. For a personal data drive, expand Sharing & Permissions in Get Info. If Ignore ownership on this volume is available, Apple suggests checking it. Only change this on a drive you control, since it affects how the Mac applies ownership on that volume.
04Back up before checking for disk errors
Section linkIf the format, free space and permissions look right but writing still fails, copy readable files to another drive before running First Aid in Disk Utility. Select the external drive’s volume and follow Apple’s instructions to work up through its containers and the device. When the drive is writable again, copy over a small test file and open it from the drive.