I got an XPS 15 from the outlet a couple weeks ago. My external hd is a WD Passport Essential SE 1TB with USB 3.0. The hard drive will work in the USB3 ports but will disconnect with even the slightest wiggle of the connector. It's a pain for me because the drive is encrypted and every time it disconnects I need to re-enter the password. Sometimes it won't be recognized again after disconnecting, and I need to keep replugging it or even reboot. The drive always works without any issues in the USB2 port or in my other laptop.
I'm calling Dell today, I'll let you guys know what they say.