Assuming you are on Windows and have installed an anti-virus product, then in Explorer right-click the disk and select the anti-virus.
This could be similar to : "Scan with Windows Defender..."
For the Mac, this depends on which anti-virus you have installed. Many of them can scan connected devices like external hard drives.
If you don't have an anti-virus, you should get one.
See for example the article
The best free antivirus platforms for Mac in 2019.