If you see the above error message in Disk Utility, it is because the JDM had not been safely ejected, which led its format to become unstructured. You can take either one of the two following measures to troubleshoot:
1. Repairing and Remounting the SSD (Not applicable to SSDs using the GUID format)
Step 1. Select “Show All Devices”.
Step 2. Run “First Aid” on the higher-level JDM.
Step 3. Run “First Aid” on the lower-level JDM.
Step 4. Mount the lower-level JDM.
2. Erasing the SSD (Applicable to all formats, but will format the SSD and erase all data from it.)
Step 1. Select “Show All Devices”.
Step 2. Run “Erase” on the higher-level JDM.
*Always safely eject your external devices to minimize the risk of data loss.