today i tried to watch movie [with vlc media player] from my external hard drive [expansion portable 500 GB], and it's play good, then pause the movie and play it again, and my Ubuntu crashed! [Ubuntu 12.04]
i had to restart the machine, so did it, but after that Ubuntu can't recognize this hard drive! i changed USB cable but it was not effective. this is my dmesg command result :

[  191.281630] usb 2-1.3: new full-speed USB device number 9 using ehci_hcd
[  191.353527] usb 2-1.3: device descriptor read/64, error -32
[  191.529115] usb 2-1.3: device descriptor read/64, error -32
[  191.704669] usb 2-1.3: new full-speed USB device number 10 using ehci_hcd
[  191.776524] usb 2-1.3: device descriptor read/64, error -32
[  191.952202] usb 2-1.3: device descriptor read/64, error -32
[  192.127772] usb 2-1.3: new full-speed USB device number 11 using ehci_hcd
[  192.534742] usb 2-1.3: device not accepting address 11, error -32
[  192.606749] usb 2-1.3: new full-speed USB device number 12 using ehci_hcd
[  193.013696] usb 2-1.3: device not accepting address 12, error -32
[  193.013906] hub 2-1:1.0: unable to enumerate USB device on port 3

note:

• I am able to hear the sound of the external drive lens.
• When attach hard drive to usb port, the status light goes on, but it's low bright and i think its power is low!
• try to mount in Microsoft Windows 7, but nothing happened.