OK.
What version of Omeka Classic are you using? There’s just one configuration setting I can think of that may be relevant here. You need to be using Omeka Classic 2.8 or newer to use it.
If you’re using 2.8 or newer, the change you’d need to make is: add the following line to the file application/config/config.ini:
storage.adapterOptions.useCopy = true