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I seem to have "solved" the problem, but it's a little bit of voodoo. I contacted Dell tech people (after interminable waits on hold), but the advice they offered was useless - they told me to uncheck "allow windows to turn device off," or whatever, in the Device Manager, but that did nothing at all. Then they told me to unplug everything from the hub and plug it into the back of the computer - I did this and it worked for a while, but then I added one thing (my USB key!) and everything got squirrely again, one of the hard drives stopped being recognized, etc. She told me it was a "power problem," which makes no sense to me since I had all the same items plugged into the add-on USB-2 card on my old Dimension 4400. There was no rhyme or reason that I could discern for why things would or wouldn't be recognized. I spent all day poking around on the web trying to find solutions but found nothing conclusive.
Eventually I did two things: First, I uninstalled every USB device, all the root hubs, everything, shown in the Device Manager and then rebooted, which caused them to be reinstalled. Second, I replaced my hub with a Belkin "TetraHub," which is advertised as "guaranteeing 12 mbps bandwidth to each port" (though that should only matter for USB 1.1) - anyway, it's supposed to be higher quality.
I'm not sure which of these did the trick, but everything's working smoothly now - I can plug additional devices in and out to any of the ports on the front or back of the computer or on the hub with no problem. But I still don't really understand what the issue was, and I'm angry that no one from Dell could offer me the slightest bit of help (or even exhibit the slightest bit of understanding of the problem). I even considered installing a separate USB card for a while - I think poor quality Dell components might be at least partly to blame.
Though everything seems to be working now, I'm quite unhappy about this. A brand-new high-end computer should *not* be giving me this kind of problem with USB ports with only 5 devices attached, three of them self-powered! This is my third Dell purchase but quite probably my last; I've had way more problems than I should have, and the much-touted service is mediocre - I routinely waited on hold for a half hour or more (each time with the announcement that the delays were "unusually" long - if they're always long it's not unusual!) to speak to someone who has no real understanding of the technical issues, but instead seems simply to be reading from a manual. Highly disappointing. It's just not worth it to save a few bucks; next time I'm going with one of the smaller companies that have people who can actually help with technical issues.
(Sorry for the vent; my frustration level with Dell, over this and a variety of other issues, has reached a boiling point).