My Seagate 4TB HDD doesn't mount in OSX El Capitan My Seagate 4TB HDD doesn't mount in OSX El Capitan on my MacBook Retina. I have been using it for the last week on this computer, but today it doesn't mount. It shows up in Disk Utility, but when I click "mount" it doesn't. 


I ran First Aid on it and it still doesn't mount.
I tried it on an iMac with the same results, but other drives work fine.
I tried to shut down, restart                - same outcome
on a different computer iMac - same outcome
different drive solid state and normal - both worked fine 
I ran First Aid in Disk Utility
When there is not something wrong with the format, it is an hardware issue.
Do you have any Seagate software installed? 
If yes, completely uninstall it, then restart the mac, it is counterproductive. 
If no: when there is not something wrong with the format, it is an hardware issue.