A situation came up with one of my clients that caused him no small consternation.
The problem is that he purchased a new 7-port USB hub from a national chain, however, when he plugged cetain devices in his had drive would disappear from his desktop. Also, he noticed that backups to that particular drive were extremely slow. After struggling for three weeks, he called me.
First thing I noticed was that he a mishmash of peripherals plugged into these hubs. I determined that at least one or more of these items had to be low speed devices and other had to be high or mid speed devices. But how to tell?
Enter Apple’s System profiler. I shut down his 24” iMac, unplugged all USB devices EXCEPT for the Apple extended keyboard and Apple mouse. I then started the machine, logged in and opened System Profiler (System Information for those on Lion.)
With System Profiler open, I selected USB in the left margin where I was able to determine what items used an internal USB bus, and which items were external buses. The three external buses (ports) operate at a high-speed of 480 Mbps (mega-bits-per-second) with the keyboard and mouse operating at 1.5Mbps, while the keyboard ports run at 480Mbps.
By connecting each peripheral in turn to either of the remaining open USB ports on the back of the iMac I was apple to determine their speeds: Epson (Seiko) printer at 12Mbps, a Dymo label printer at 12Mbps, an Iomega Solo drive at 480, an Epson scanner at 480, a Seiko camera at 12, a multi-card reader at 12Mbps, an 8-port , powered, USB hub at 480 and a NEW 7-port, powered USB hub at 480.
To solve his speed problems, I plugged all his slow-speed devices (the printers, camera and card reader) into the older, 8 port hub which I then plugged into one of the available USB ports on the iMac. I them plugged in the 7-port hub to the last USB port on the iMac then plugged the two high-speed devices (SOLO and scanner) to the 7-port hub.
Here’s why I did this: a HUB only transfers as fast as the slowest device connected to it, even when daisychaining devices. Thus, if you plug a USB mouse running at 1.5Mbps into a high-speed hub performing (theoretically) at 480Mbps, the hub actually falls to 1.5Mbps and all transfers slow to a crawl.
Here’s something more interesting: From my experimentation and observations (speculation?) the aluminum Apple keyboard must be SWITCH, not a hub. A high-speed device plugged into one of its ports will transfer at high speed (though, apparently, not full speed) at the same time that a slow speed mouse is plugged into the other port at the same time! Very few keyboards will do this.
Anyway, with the slow speed devices on one hub and the highspeed devices on the other hub and both powered and plugged into separate iMac ports, his machine ran properly andhis backups speed up ten-fold!