I've seen this happen for years with no "one thing" being the cause. I have run into folk that think it's a driver or the maker but let me share I have ZERO PROBLEMS with this today.
However I know to look at the various control panels to remove, uncheck power saver settings, change the drive to not spin down (if that's what I want) and a few other things.
Sadly, I've never found it to be one thing. One time it was the same old USB power issue. That is, using a wall wart to power the drive fixed it.
Then again, I run into folk that want Dell to fix it so the wall wart is not required.
Bob
I am having random disconnection issues with the 2 USB 3.0 ports on my newly purchased XPS L702X.
It is completely unrelated to the device and occurs at no specific interval and is only recoverable with a reboot.
Based on a Dell Support Forum - XPS 15 - USB 3.0 Problems, this issue has existed since the 1st generation release late 2010 of the current XPS series and appears to affect many users.
Based on feedback, consumers who contact Dell for resolution end up with no resolution despite motherboard/daughterboard replacements.
Also, a Renesas controller driver upgrade or rollback does not cure the issue either.
I am starting this discussion to find others who have encountered this issue as well and have either found a resolution to actually using the 3.0 ports or who may have not yet spoken up... This issue has gone on far too long without resolution and in order to get Dell to take this defect seriously, everyone affected needs to read the forum and take part in the descussion,
Please post any information you have... even if you have an XPS Laptop current model 1st and 2nd generation without 3.0 issues... you too can help find the solution.
Thanks

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic