Brand new external hard drive formatted exFAT is mounted read only

If your exfat drive shows up as read-only, then all you need to do is unmount the drive and then use the "mount_exfat" utility to mount the drive. Once you do this, and right click on the drive → "Get Info" will tell that you have custom access to the drive, instead of "read-only". Here are the steps:

1. Open the terminal on your mac.

2. Type diskutil list and you will get a listing like this:

   /dev/disk0 (internal, physical):    #:                       TYPE NAME SIZE       IDENTIFIER    0:      GUID_partition_scheme                
   *500.3 GB   disk0    1:                        EFI EFI                     209.7 MB   disk0s1    2:                 Apple_APFS Container disk1         500.1 GB   disk0s2
   
   /dev/disk1 (synthesized):    #:                       TYPE NAME        SIZE       IDENTIFIER    0:      APFS Container Scheme -              
   +500.1 GB   disk1
                                Physical Store disk0s2    1:                APFS Volume Untitled - Data         407.2 GB   disk1s1    2:           APFS Volume Preboot                 82.4 MB    disk1s2    3:           APFS Volume Recovery                528.5 MB   disk1s3    4:           APFS Volume VM                      3.2 GB     disk1s4    5:           APFS Volume Untitled                10.7 GB    disk1s5
   
   /dev/disk2 (external, physical):    #:                       TYPE NAME SIZE       IDENTIFIER    0:      GUID_partition_scheme                
   *5.0 TB     disk2    1:                        EFI EFI                     209.7 MB   disk2s1    2:       Microsoft Basic Data Backup Plus             5.0 TB     disk2s2
   

3. In the listing above, we notice that /dev/disk2s2 is our external drive. Unmount the drive by runningsudo umount /dev/disk2s2`.

4. Now create a directory in in /Volumes by running sudo mkdir -p /Volumes/<name of your volume>

5. Mount the hard drive to this directory your created by running sudo mount_exfat /dev/disk2s2 /Volumes/<name of your volume>.

Once this is done, you should be able to create new folders and write to your drive.

Being mounted in read-only mode is necessary for fsck (which is short for File System ChecK) to run; it ensures that no processes other than fsck are tampering with the data on the disk.

If a disk is mounting read-only it's because the operating system detected a problem with it which requires repair. If you have access to a Linux or Windows machine they may be able to run the disk repair process more quickly, but regardless, something needs to go through the whole disk and repair whatever damage may have occurred by the unclean disconnection.

Simply waiting for there to be "no processes" is also insufficient to ensure that a disk is safe for removal; the operating system takes some time to flush out its write caches and other performance-enhancing processes that allow you to get more work done while the actual disk operations happen in the background. It's possible that the files were not fully transferred; hopefully you have backups of them (which is a good policy to have in any case).