Opus itself would not tell the drive to dismount and essentially just opens files and writes data to them.
Maybe something Opus does slightly differently to Explorer is triggering a bug in the USB drivers/firmware/etc. (Most things are only tested with Explorer, so they'll usually work with that, of course.)
Changing the Copy Buffer Size which Opus uses will sometimes help with buggy USB devices, so that may be worth a try. (Preferences / Miscellaneous / Advanced: copy_buffer_size)
You can also set up a button in Opus that copies files using the shell (i.e. the same way Explorer does) as a workaround: Copy files via the shell (Windows Explorer)