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:


Open the terminal on your mac.
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

In the listing above, we notice that /dev/disk2s2 is our external drive. Unmount the drive by runningsudo umount /dev/disk2s2`.
Now create a directory in in /Volumes by running sudo mkdir -p /Volumes/&lt;name of your volume&gt;
Mount the hard drive to this directory your created by running sudo mount_exfat /dev/disk2s2 /Volumes/&lt;name of your volume&gt;.


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