USB 3 Expectations (Solved - sort of)

Hi all
Recently moved to a new machine (specs below). In initially setting up I shunted data across via USB 3 with rates typically around 125 mb/s. However, since then, performing backups to the same external drive, I rarely achieve much above 60 mb/s and sometimes as low as 40 mb/s. I've tested in both directions and get the same each way.
 
I'm wondering what could have changed, whether other folk see the same slow-ish speeds with USB 3, or if my off-the-shelf Seagate Expansion drives are poor. Interestingly, imaging with True Image to the same drive, is blisteringly fast compared to USB 2.

Not whinging here as I’m grateful for the added speed above USB 2 - and the move to the new system has been thankfully without issue. However I’m very curious as to why the speed would have dropped since day one and be so much lower than the advertised spec.
 
Many thanks & happy New Year!
David
 

Hi Mettelus. Thanks for that.
However I have discovered that if you enable Write Caching for the external drive, you double the transfer speed. After an initial burst of enthusiasm, it is settling down to a stable 116 MB/sec. By turning off the Buffer Flushing you get an initial burst that is dramatically higher but it then settles down to 116 MB/sec fairly quickly. This is good enough for my purposes. Perhaps this is standard practice that I'm not aware of?
System/Device Manager/Rt click Disk Drive/Properties/Policies/Better Performance/Enable write caching/
Cheers
David

Thanks Spacealf. Yes, I have a UPS but you've reminded me to replace the batteries.
Cheers
David