First of all, let me say that you can probably solve your immediate problem in a flash using:
Search->Search and Replace->Field to search->Multimedia filenames->Search for->D:\Documents\Legacy->Replace with->D:\Documents\Rootsmagic
(ie. search the list of media file paths in the database for any having [OLD Default Media Folder] in the path name and replace all occurences with [NEW Default Media Folder] path name.)
I move my Media folder from Documents\Legacy to Documents\Rootsmagic.
The Media folder contains 2,800+ images.
I'm thinking you most likely mean you moved Documents\Legacy\Media folder and ALL subfolders to Documents\Rootsmagic\Media ?
And I'm assuming you mean the Media folder and ALL its subfolders contain 2,800+ images, since you mention a \Censii and \Arms subfolders ?
If I simply use Tools, Fix Broken Media Links, select the entire Drive, it will actually find 7 media files in the Documents\RootsMagic\Media\Arms subdirectory, but otherwise freezes at that point.
Subsequent step here would be to manually edit and correct those 7 file paths in the Media Gallery and then rerun the Fix Broken Media Links to see if/where it freezes again/next.
If I select <A Folder> and tell it my Media folder (Documents\RootsMagic\Media) it finds 0 missing files.
Here's where you lost me. I believe if you select <A folder> Rootsmagic should automatically (from the Program Options->Folders setting) "already know the new default" and offers to open D:\Documents\RootsMagic\Media folder as the starting place for a search (one can instead choose to select an explicit parent or sub folder from any available drive -or- an explicit drive letter to search from a drive root folder down into the subfolders).
(Searching the entire drive means it's got to go through 81,244 files before it reaches the first RootsMagic directory.)
Yeah, that's the longest way, but you most likely wouldn't still be here in the forum hours/days later scratching your head ;O)
Under Tools, Program Options, Folders, I have told it my Multimedia files are at Documents\RootsMagic\Media.
It won't look where I've told it to look.
This sounds like an innocently mistaken view. Why do I say this? Because I have used the dialog to choose various different starting points for a search with no failure.
It doesn't recognize the [Default Media Folder] as the one I've told it to use.
My response immediately above ...applies here too. If it didn't work there would be 100's of complaints.
And it crashes after locating 7 of the 2,800+ files I know are there.
What am I doing wrong.
This is the problem area. Could be corruption in the database. Run File->Database Tools (all 4 top to bottom). As an isolation test, create a fresh new empty database and dragN'drop EVERYBODY to this new database. Then try your Locate Missing Media Files again from this newly created copy. If it's still failing, perhaps you should file a trouble ticket and offer your database for technical support scrutiny. Hang in there!