Exactly same issue here.
I was shocked today when I found out the difference. I'm using a USB 2.0-connected LaCie 2GB drive as a secondary drive to my dual-SSD mid-2011 Mac mini Server. Using BlackMagicDesign's Disk Speed Test I came to following results:
- directly connected to the back of the Mac mini: 29MB/sec write, 33 MB/sec read
- connected to the TB Display: 7.5MB/sec write, 11.7MB/sec read.
I'm running OS X Mountain Lion 10.8 on the mini, and the tests were repeatable, after fresh power down - reboot cycles.
That's not what I bought the Thunderbolt Display for...
Any ideas, anyone?
I have the same issue on a new TBD. The USB ports on the TBD are slow (13-22 MB/sec.). I also noticed that if I have the Apple super drive connected it runs even slower (7-10). Looks like the Super drive is consuming some amps. or the USB buss is setting speed based on the Super drive speed. This defeats the purpose of having USB ports on the TBD.
MacBook Pro Retina
Plugable USB 3.0 SATA Hard Drive Lay-Flat Docking Station (Latest ASMedia ASM1053E Chipset)
Seagate Barracuda 3 TB HDD ST3000DM001
Disk Speed Test by BlackMagicDesign
Plugged directly into the MacBook:
Write/Read: 165 MB/s 172 MB/s
Plugged into the Thunderbold display
Write/Read: 7 MB/s 12 MB/s
A difference of almost 20-1 !!!!