Every SSD is mounted as read-only on my 15" 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
$ 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.