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At home, I plugged the USB stick to my Linux computer and, sure enough, I could see all my files plus the malware-generated ones. The malware had inserted copies of itself in every folder and sub-folder while making my original files invisible to Windows. What a nostalgic feeling!
After I deleted all the copies of the malware, I searched for the way to make my files and folders visible again on Windows.
It turns out that Windows does it like this:
attrib -H -R -S /S /D F:\*.* (F stands for the letter assigned to your USB).
Well, it seems that Windows malware keeps being as effective as it was back in the times I used that OS. Some things never change!