Does Directory Opus use OS copy routines?

I am copying 100,000 files to my FAT32 microSD using an USB-to-SD Card reader. On the SD Card, directory names are converted to UPPERCASE after copying.

Does Directory Opus use the Windows Copy-routine, or is it using a different method?

Opus normally uses its own copy routines, but that has no effect on filename case and I doubt the method of copying has anything to do with what you are seeing with the case of names on your SD card.

The SD card may be formatted with a filesystem which only supports uppercase characters.

Explorer will normally convert all-uppercase names to lowercase, to hide the ugliness of old DOS filesystems.

Opus can do that as well, but it is optional. (Preferences / Folders / Folder Display / Modify display of all-uppercase filenames.)

If you are seeing a difference, it is probably just that, and the names are all uppercase no matter what copies the files to the SD card.