I can confirm the bug. And it started after the upgrade to 13.04 - 64bit on a Dell XPS-8500. It was not happening on 12.10.
Only affects USB 3.0 ports (2.0 safely removes ok).
The only way to unmount a USB hard drive in my case is clicking on the arrow icon on Nautilus window. If I click on Safely Remove, both on Nautilus or on launcher icon, the drive is re-mounted.
I can confirm the bug. And it started after the upgrade to 13.04 - 64bit on a Dell XPS-8500. It was not happening on 12.10.
Only affects USB 3.0 ports (2.0 safely removes ok).
The only way to unmount a USB hard drive in my case is clicking on the arrow icon on Nautilus window. If I click on Safely Remove, both on Nautilus or on launcher icon, the drive is re-mounted.