If your Mac can't save files to an external drive
If you can read files from an external storage device that is directly connected to your Mac, but you can't write files to it, try these solutions.
Check storage space
If the storage device doesn't have enough free storage space to contain the file, your Mac will let you know. To make more storage space available, delete one or more files from the storage device. You might also be able to save space by compressing files.
Check the format
Select the storage device in the Finder.
Choose File > Get Info from the menu bar, or press Command-I.
In the Info window that opens, the listed Format should be one of the formats available in Disk Utility:
APFS or Mac OS Extended: These are Mac formats.
MS-DOS (FAT) or ExFat: These are Windows-compatible formats.
Disk Utility does not support NTFS format, which your Mac might be able to read from, but not write to.
If the storage device isn't using one of these formats, use Disk Utility to erase and reformat the storage device.
Check Time Machine
If you set up Time Machine to use this storage device for Time Machine backups, then you can use it only for Time Machine backups, not for other files.
Check permissions
Choose File > Get Info from the menu bar, or press Command-I.
In the Info window that opens, open the Sharing & Permissions section at the bottom and be sure that the “Ignore ownership on this volume” checkbox is selected. To change this setting, you might need to first click the lock
and enter an administrator password.
Check for errors
Use Disk Utility to find and repair disk errors. If Disk Utility finds errors that it can't repair, or if none of the other solutions work, use Disk Utility to erase and reformat the storage device.
