Files with mcg file extension can be found as backup files from DX-client– HDD viewer software.
MCG file extension - DX-client backup
What is mcg file? How to open mcg files?
File type specification:
The mcg file extension is used for backup files used by DX-client– HDD viewer software.
Apparently, same extension was also used by PC Viewer D6, which is a program very often distributed with some DVD recorders. Different format, just has same extension.
This mcg file type entry was marked as obsolete and no longer supported file format.
This type of file is no longer actively used and is most likely obsolete. This is typically the case for system files in old operating systems, file types from long discontinued software, or previous versions of certain file types (like documents, projects etc.) that were replaced in higher versions of their original programs.