Hi: My OS is 10.5.7. I have an external hard disk, formatted as Mac OS Extended (Journaled), connected to my Mac machine through USB cable. I have used it for a while, but now it is 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. Luckily, in my work place there is a Mac with OS 10.4, so I had plugged the disk into the 10.4 machine and it worked!!! I did checked "System Profiler" 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