You problem is incredibly straightforward.
In Linux Systems, the file extensions for a MyISAM table
tablename must be
The file extensions are case-sensitive
Windows does not check case-sensitivity for
.myi file extensions, so a MyISAM table in a Windows MySQL instance will allow for this. Such is not the case with Linux.
In your case, let's look at the
UsersInGroups table. You evidently have two tables with two different filenames and/or extensions:
You will have to rename the files in the OS so that there are only
.MYI extensions. You can then query each table using the case-sensitive name
If you simply want to replace one with the other. delete the old files, keep the new files, and make sure you only have
.MYI extensions. BTW
.frm must always be lower case.
If you are using dbForge to make a backup of MyISAM tables from a Windows machine and you restore that backup on a Linux machine, you must always remember to change the file extensions after restoring from backup.
I talked about this 5 years ago (How can I move a database from one server to another?)