Copying a file to a removable media is almost old fashion, but there are people who still use such technique.
Malware attack

There is this type of malware that, if attacks a PC, will make files copied on a removable media hidden.

Even if checking the content of such Media shows there are files on it, a user will still not be able to see the files present on such media.

Following the below listed procedure can help restore the visibility of such files:
Update and scan the PC with a good antimalware

The first aid to fixing a PC infected with a malware is to scan such PC with an up to date Antivirus/antimalware software.
Scan the infected media

Once the PC has been confirmed to be free from malware, the next thing is to scan the affected media.
Command prompt

Open the command prompt with super user privileges and enter the following command:

attrib -r -h -s -a /s /d G:*.*

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