It looks like a very careful assessment of the USB capabilties may be in order. The USB link can be a critical part of a SheevaPlug set up, and if USB components have a variance either in meeting specifications and/or performance, then these facts should be considered when attaching a USB device to the SheevaPlug. I've been working under the assumption that if a product says it meets USB 2.0 specifications, then everything should work?
Well, a read of http://www.linux-usb.org/FAQ.html
has caused me to wonder if I should scrutinize the specifications of USB devices. For instance:
First, you need to know what kind of USB "Host Controller" hardware you have. Mainstream hardware has one of three kinds, named after the hardware register-level "Host Controller Interface" (HCI) they implement. ...
"Universal" HCI (UHCI) ... doesn't do very much in hardware, which makes the software do more work (and need more memory)
"Open" HCI (OHCI)... does quite a bit more of USB in hardware.
"Enhanced" HCI (EHCI)... the only kind used to talk to high speed devices.
As I indicated in my initial posting, mixing the Targus Hub with a Verbatim thumb drive caused problems. I've learned Verbatim sells a USB hub (# 95258); however, they do not certify it for Linux. I have pending an inquiry with Verbatim's customer support asking for an explanation of why they do not certify their drive for Linux. My thought in buying a Verbatim hub was I might finesse the problem I previously experience by using the same manufacturer's, i.e. Verbatim, components on the hope that they play nicely together.
I'm guessing that a hub has to be a traffic coordinator among its children and the device it is attached to. I can imagine how problems could arise if everyone doesn't use the same protocol and/or have compatible specifications. Add to that, the design of the kernel's device driver.
For reference, here's what I found that relates to the USB interface in the 1.0 documentionation, SheevaPlug Development Kit - Reference Design-Rev. 1.0.pdf:
Figure 1.7: Block Diagram (sheet 10/21) has "Usb 2.0 with Phy interface"
Section 2.8 Drive Connection
USB 1.1 or USB 2.0 are supported
3.4.3 USB 2.0
88F6281 contains a Universal Serial Bus 2.0 port includes an embedded USB 2.0 PHY. The USB interface can support either Host or Device mode. The SheevaPlug design supports HOST mode only. There is no strapping provided by the SheevaPlug board to switch between HOST or DEVICE USB mode. USB 2.0 port feature 480 Mbps, 12Mbpsm and 1.5Mbps data rate; bit stuff error detection, NRZ bit stuffing, and built in FS/HS termination signaling.
Am I over-analyzing here? If people have attached hubs and have not had problems, it would be helpful to so indicate. The lack of response to my initial posting has me wondering if people are not using hubs.
As an aside, I was looking at Corsair's 64 GB thumb drive (Model CMFUSBHC-64GB) on NewEgg's web site and read an interesting comment:
It seems many people don't understand that when transferring Data directly from a USB to USB it's going to be slow regardless what the Flash drive capacity is. My Best advice to you will be to go from Flash Drive to HDD to Flash Drive you will drastically reduce your transfer time. Example: 14gigs of data on a USB Drive would have taken me 3+ hrs transferring directly to USB Drive. It actually took 20 mins Total (12 mins from USB Drive to HDD then 8 mins HDD to USB Drive) . Understand that going from USB to USB is not a ideal way of transferring data.