How to mount an external drive (APFS) as read only?

[canhaz]

Been tearing my hair out over here trying to figure out why it's so damn hard to mount my USB backup (Carbon Copy Cloner was used) as "read only"?

I can't even do it via command line

sudo mount_apfs -o rdonly /dev/disk1s1 /Volumes/Backup mount_apfs: volume could not be mounted: Operation not permitted

Not seeing any option in Disk Utility either to mount it read-only. Regular (r/w) mounting and unmounting works fine via Finder or Disk Utility.

Anyone managed to do this successfully under Catalina?

 

[Taz Mangus]

Been tearing my hair out over here trying to figure out why it's so damn hard to mount my USB backup (Carbon Copy Cloner was used) as "read only"?

I can't even do it via command line

sudo mount_apfs -o rdonly /dev/disk1s1 /Volumes/Backup mount_apfs: volume could not be mounted: Operation not permitted

Not seeing any option in Disk Utility either to mount it read-only. Regular (r/w) mounting and unmounting works fine via Finder or Disk Utility.

Anyone managed to do this successfully under Catalina?

Maybe try using the diskutil command.

sudo diskutil mount readOnly -mountPoint <path> <device>

Mount a single volume. If readOnly is specified, then the file system is mounted read-only, even if the volume's under-lying file system and/or device and/or media supports writing; even the super-user may not write to it; this is the same as the rdonly option to mount (8). If a -mountPoint is specified, then that path, rather than the standard path of /Volumes/VolumeName, will be used as the view into the volume file content; a directory at that path must already exist.

EDIT: I just realized something. Did you create the destination directory /Volumes/Backup before you executed the mount command?

 

[canhaz]

Excellent. That worked like a charm, ty. To be honest not sure why I didn't try that first.

Just a headsup for anyone else reading, seems you also needs to unlock the drive first if it's encrypted as follows:

diskutil apfs unlockVolume /dev/disk1s1 -nomount
Passphrase:
Unlocking any cryptographic user on APFS Volume disk1s1
Unlocked but did not mount APFS Volume