SSD always mounts as read-only on Mac OS Mojave Every SSD is mounted as read-only on my 15&quot; Macbook pro (with touchbar). For this reason, time machine doesn't backup anymore. IIRC, this all started happening after upgrading from 10.14.x to 10.14.x+1 - I don't remember exactly which version but it was several months back. The same issues used to happen on upgrading to catalina. That's why I had gotten a new laptop with mojave last year.
In the past, I have tried to repair the drive, filesystem and try the mount command on the commandline to get it to mount in rw mode. It would be great if the drive mounted rw the first time.
$ diskutil list
/dev/disk0 (internal):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                         500.3 GB   disk0
   1:                        EFI EFI                     314.6 MB   disk0s1
   2:                 Apple_APFS Container disk1         500.0 GB   disk0s2

/dev/disk1 (synthesized):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      APFS Container Scheme -                      +500.0 GB   disk1
                                 Physical Store disk0s2
   1:                APFS Volume Preboot                 47.2 MB    disk1s2
   2:                APFS Volume Recovery                510.8 MB   disk1s3
   3:                APFS Volume VM                      5.4 GB     disk1s4
   4:                APFS Volume MacOSMojave500GB        480.6 GB   disk1s5

/dev/disk2 (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *1.0 TB     disk2
   1:                        EFI EFI                     209.7 MB   disk2s1
   2:                  Apple_HFS SANDISK1TB              999.9 GB   disk2s2

On doing
$ sudo mount -v -u -w /dev/disk2s2 /Volumes/SANDISK1TB

the disk is mounted in rw mode. However, only root can write to it.
Thanks for your time.
I wrote a bash script that I execute every time after inserting the drive
function mountTimeMachineDriveRWSANDISK1TBSSD() {
    diskutil mount /dev/disk2s2 # disk2s2 is from the output of diskutil list
    sudo mount -v -u -w /dev/disk2s2 /Volumes/SANDISK1TB # disk2s2 is from the output of diskutil list
    sudo chown myuser /Volumes/SANDISK1TB
    sudo chgrp mygroup /Volumes/SANDISK1TB
}

Now the disk is writable by the current user. Time machine can backup to this disk too.