Hi James,
Sorry I missed your question earlier. The .rmdb file is the ReefMaster DataBase file, and is a single file on disk. This file will have the name <work_workspace_name>.rmdb and is located in a folder with the same name, minus the ".rmdb" extension. The location of this folder varies - you choose it yourself when creating a workspace. The default is likely to be your
Documents folder. If you are not sure where your workspace is located, then start up ReefMaster (or click the
Open Workspace button) and hold your mouse pointer over the name of the workspace in the recent workspaces list. The full path will appear as a tooltip - see the image below.