You do not need to have all your disks connected at all times in order to use catalogs. If you have generated proxies for the photos in your catalog, you can still see thumbnails in catalog contact sheets. Offline or "missing" media will be indicated with a yellow circle under the thumbnail.
A green circle indicates that the images are active or stored on a drive connected to your computer. A hollow circle (the color matches the background color of your contact sheet) indicates that the image is not currently in a catalog or that the file was moved outside of Photo Mechanic Plus and is no longer in sync with the catalog.
You can still preview images that are missing if the catalog has not moved. You cannot currently add metadata. Any changes you make to the photos outside Photo Mechanic Plus must be synced using Catalog Sync when the media is online.
Find all your missing images
To select all your offline or missing images, go to Edit>Select missing.