There are two common reasons for an external drive to no longer appear within the Mylio Photos application. If you have recently upgraded the operating system on your computer, Mylio Photos might have lost the permissions to access removable drives. You can check for this on macOS under:
System Settings → Privacy & Security → Files and Folders → Mylio.
Correcting this setting will help with both new drives you want to add to Mylio Photos, or existing drive devices that are no longer showing up in the app.
For existing devices that do not appear in Mylio Photos, the other thing to try is a Catalog Repair on the external drive. The catalog is a file on each device that holds information about your library, such as how many photos you have and where they are stored. If this becomes damaged, the app will not function properly. Mylio can use a catalog from one of your other devices to sync a new catalog to the device. To perform a catalog repair, please take the following steps:
Select Help in the menu bar, then choose Show Support Options on Startup
Quit and reopen Mylio Photos to display the Support Options Window
Choose the "Repair a Catalog" option
Choose to repair the catalog corresponding to the missing device
Mylio will check for any errors. Even if no catalog errors are found, repair the catalog anyway
Mylio will attempt to repair the drive's catalog using the catalog from your other devices. After the process is complete, Mylio should open normally and your drive should become available.