although there are tons of questions regarding USB 3 transfer speeds, I will ask another one.
I have two external hard disks used for data backup. Both are Transcend StoreJet, lsusb shows the following details:
Bus 004 Device 005: ID 174c:5106 ASMedia Technology Inc. ASM1051 SATA 3Gb/s bridge
The PC is a fairly recent Intel Core i5-4440 running @ 3 GHz.
Operating system is Ubuntu 16.04.
Internally, the disks appear to be WD Green disks. See https://www.heise.de/ct/ausgabe/2015-15-Test-Robuste-externe-Festplatten-2719434.html for a (german) test of the disk.
The disks are expected to produce ~ 100 MByte/second data transfer when plugged into an USB 3 port as per that test.
They do so, if run in single mode (i.e. transferring data from an internal disk to the USB 3 disk) e.g. using an rsync job.
rsync -avuz /home/marc/<some_dir>/ /media/marc/disk2/
However, if I plug in two of those guys and transfer data between them, the transfer rate drops to ~ 30 MByte/s.
rsync -avuz /media/marc/disk1 /media/marc/disk2
It does not matter which (USB 3.0) port I use (e.g. both connecte to two adjacent ports on the front panel or "directly" to the ports on the back (on the motherboard) or one in front and one in the back.
I can verify, that they are detected as SuperSpeed disks in all cases.
Although this may not be Ubuntu specific -any way to analyze this further (I looked at iotop, dmesg, lsusb already) or is this a known USB driver issue?