I have a My Passport Ultra from maybe 2014ish that’s not mounting to either my own macbook pro (2013, currently running El Capitan) or my new Mac Pro (2017, currently running Sierra). The light appears and it whirs like it’s running, but it does not show up in the finder or in disk utility. It connected to my macbook when I first got it, and I didn’t run an update in between the last time I used it and the first time it wouldn’t mount. It still mounts to my girlfriend’s macbook (sierra), but only if I hold the usb in for a little bit before letting go.
edit: as an interesting tidbit, when it’s plugged in and I’m trying to mount it, my internet connection drops, though correlation doesn’t imply causation
