Poor transfer rate on Windows Server 2012 R2 on USB3 External HDD

I've upgraded from Windows Server 2008 R2 to Windows Server 2012 R2 on a physical server built on a 2011' Asus motherboard. Windows 2008 was known to not support USB3, and that was an issue since years because we frequently need to transfer large VHD's from the host to an external 2.5" HDD to take VMs with us for demo purposes. I was hoping with Windows 2012 R2 that we could get fast transfer rates, but still the transfer speed printed on the Windows Explorer dialog is 22,8 Mb/s. The same external HDD connected to a Dell laptop speeds up to 78 Mb/s. (I compared copying 2Gb file TO the external HDD). What am I missing?