I used my Passport external hard drive as normal last Friday and ejected when I was finished. Now when I connect it makes an awful high pitched sound, very similar to a modem dial-up for those of you who remember those. And it doesn’t show in Finder. The connection seems ok, I’ve tried multiple ports on my Mac and you can hear the hard drive motor power up each time. The noise is horrendous and it just doesn’t connect.
You may try a different USB cable and avoid USB hubs to directly connect to a USB port, if the drive show up in the Disk Utility; run First Aid.