My mac is automatically formatting usbs?

I have a feeling that my mac is reformatting disks automatically. For instance, I connected my TDK 16GB usb to my windows machine first to see what was on it (it worked), and then I plugged it into my mac to put a windows software on it (the windows 10 wifi driver isn't working), and then when I plugged it into my windows machine, it said this drive is broken, I then checked the format via the disk management tool on windows, and it said it was a RAW type, but then I plugged it back into my mac, and it said it was MS-DOS (FAT-32)?!

Are there any known macintosh viruses that reformat/corrupt hard-disks automatically?! I have used uTorrent to download a couple tv show episodes recently, could I have got a virus from that?! I checked the file types before I downloaded, they were .mkv. Maybe I got it from a website?

Is this a known virus/issue or has this happened to anyone before?

I'm using el capitan on a 2015 15" macbook pro.

Is there any type of free virus scanning software for mac? Even if this isn't a virus, I'd like to double check!

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