Dell XPS 15z laptop's USB 3.0 ports does not work with at least one USB 3.0 external hard disks out there; Toshiba Canvio 1 TB USB 3.0 portable external HDD.
Check the Dell forum for the details of the problem.
XPS 15z laptop USB 3.0 problem
The hard disk works fine with the USB 2.0/eSATA combo port at USB 2.0 speed. (~30MB/s). The hard disk is not recognized at the USB 3.0 port. When I plugged it, the hard disk blinked with blue light continuously and nothing happened. The same HDD gives around 90 MB/s data transfer speed in my Desktop's USB 3.0 port indicating that the hard disk works fine.
So far, the mother board has been replaced and that didn't fix the problem. A newer laptop also had the same problem. It appears to me that the USB 3.0 port does not provide sufficient power to such external hard disks. To test this, I purchased a cheap USB split Y power adapter cable and plugged the external hard disk to both of the USB 3.0 ports at the same time; the additional plug is expected to provide the extra power to the hard disk. My suspicion was right; now the hard disk is recognized by Windows. Since the split cable I bought supported only USB 2.0, I get only about 30 MB/s speed.
Now one option for me is to get a USB 3.0 power adapter split Y cable if I need the super speed with this hard disk, which is about three times of the USB 2.0 speed in real life. Three times speed is useful if you have huge data to transfer; say you'll be done in 20 minutes instead of 1 hr.
Is it a design flaw from Intel in manufacturing such a motherboard? Is it Dell's fault in not disclosing this potential incompatibility to the customers when advertising USB 3.0 as a specification of the laptop? Or is it a problem from Toshiba in making a power hungry external hard drive? Who sets the standards for USB 3.0 power specification and requirements? (if there is such a standard).
I've no idea.
Update on April 18th 2012
Previously, I reported that the drive works with USB 2.0 Y split cable. It could transfer files in USB 2.0 high-speed (~30 MB/s) from that connection. But to get the USB 3.0 super-speed (~90 MB/s), I purchased a USB 3.0 split Y cable from Amazon.com. As I expected, the drive did work this time with USB 3.0 transfer rate. The problem is solved. But two of my USB ports are gonna be occupied if I want USB 3.0 speed. Well, at least it works. So finally, the problem seems that either the Dell XPS 15z doesn't provide sufficient power to the USB Hard disk or this specific USB hard disk is under powered from a single USB port.
Hey, Dell, Intel, Toshiba....who is ready to take the responsibility?