I have a MacBook Pro 2019, My seagate 1TB hard drive would not mount for a while..... I have a thunderbolt -usb adaptor connected and thought it might have originally been a poor connection due to cable, after restarting my Mac in safe mode and restarting etc etc nothing appeared to work. but after leaving the device connected for approx 20mins the hard drive and all its files have appeared. just thought id share this anyway.