my OS is 10.5.7. I have an external hard disk, formatted as mac os extended (journaled),
connect to my mac machine through USB cable. 

I have used it for a while, but now it not working.

error showed in /var/log/system.log.

USBF: 2521.384 AppleUSBEHCI[0xa515800]::Found a transaction which hasn't moved in 5 seconds on bus 0xfd, timing out! (Addr: 0, EP: 0)

.....

......

kernel[0]: USBF: 2588.616 [0xa6baa00] The IOUSBFamily was not able to enumerate a device.

but luckily, in my work place there is a mac with OS 10.4, so i had plugged the disk to 10.4, it worked!!!

I did checked "System Profile" on both machines (10.5, 10.4). under usb section , for 10.5 it shows USBEHCI, under 10.4, it shows USBOHCI.

I am not a expert on USB Device , can any one tell me why 10.5 cause the problem.

**it was worked under 10.5**