USB Bus - Any practical limits ?

[dave261266]

Hi Guys,

Just thought I'd throw this one out there as I have been having some strange problems with my Imac over the past couple of months and I'm getting rather irritated by it.

Kit is as follows

24" 2.8Ghz Imac 4Gb Ram (Internal 320Gb drive)
USB connection to a Drobo Array
USB Connection to a Kingston 7 port USB hub connected to 6 external USB HDD's (5 x 320Gb, 1 x 750Gb)
USB Connection to a Kingston 7 port USB hub connected to 1 external USB HDD plus camera docking station, iphone docks (x2), USB keypad

What's happening is that all of a sudden programs will start not wishing to shutdown, no forceful kills will work. In every instance I have noticed that time machine gets stuck in a "preparing Backup" mode but I am unable to determine if this is the cause of a symptom. Other things will continue to work but eventually it will slow down to the point of being unusable or the GUI manager will become unresponsive and make the machine useless. At this point ssh still works and I can have a look around at the command line level but the processes that it cannot kill are left in brackets.

Trawling through the logs yesterday I noticed some USB timeouts and also some io errors on the time machine backup disk. I have checked the time machine disk and disk tool showed no errors. As a test I have turned time machine off for the moment to see if that helps but it will be hard to tell as the box can be up for a week before this happens.

Has anyone had any behavior similar to this ? Could it be related to the disk io errors that I am getting. Also I would have thought that if I am getting io errors on a disk then disktool should be able to find them ? Thoughts ?

Regards

Dave