The drives are trying to draw too much power from the USB Bus.

USB 3.0 DRIVES are notorious for this.

USE a powered Hub with wall socket or USB Y Cable to tap extra power.

http://www.amazon.com/Apricorn-Power-Adapter-Cable-AUSB-Y/dp/B000JIOHDE

The drives are already plugged into powered USB hubs and they are not USB 3.

It is not a USB problem: it is a driver problem.

In device manager every device appears, but when I right-click on any of them, here's what it says in the General tab:

Windows cannot load the device driver for this hardware. The driver may be corrupted or missing. (Code 39)

I Googled Code 39 and already investigated the upper and lower limits registry issue, but I do not have those entries.

When I uninstall any of the devices and reboot, Windows tries to reinstall the driver but it always ends in an error message.

Then your powered Hubs are not putting out enough current.  Western Digital and other External Drives Draw too much power  aka more than 500ma for USB2 and more than 900MA for usb 3.

http://www.newegg.com/Product/Product.aspx?Item=9SIA07Y0SH3206

http://www.newegg.com/Product/Product.aspx?Item=N82E16812200791

You did not say what make and model the drives were. Either way this is not a Dell Support Issue.

THERE ARE NO Additional Drivers for XP/VISTA/7

If these are SD Readers and its XP then there is a hotfix.

http://www.microsoft.com/en-us/download/details.aspx?id=17175

There is also a Driver for XP for EXFAT

http://www.microsoft.com/en-us/download/details.aspx?id=19364

And the final possibility.  The drives have been dropped and are now dead.:emotion-9:

You did not provide ANY information as to Which Model Dell and or What OS and service pack.

Then lets refresh the USB Stack

Control Panel - Device Manager - USB Serial Controllers - RIGHT CLICK on EVERYTHING and UNINSTALL
all except the category itself - REBOOT - this refreshes the drive and the USB stack

This KB shows the XP how to and the Vista method is identical
http://support.microsoft.com/kb/310575

----------------------------------------------

If needed try both of these :

Tips for solving problems with USB devices - and a Mr Fixit
http://windowshelp.microsoft.com/Windows/en-US/Help/c39bd203-f729-47a4-8351-83291e13c8a81033.mspx#EGB

Hardware devices not detected or not working - Mr Fixit
http://support.microsoft.com/gp/hardware_device_problems

These are stationary internal drives that have been in the same external cases and connected to the same hubs for several years with no issues.

This is unrelated to the hubs because even my Lexar USB Flash drive (plugged directly into the machine) is showing the same issue.

I already uninstalled all of them and rebooted.

Win 7 Pro tries to install the drivers and it always fails.

System:

Dell XPS 9100, Win Pro 64X, SP1

Intel Core i7, CPU 930 @ 1.80 Hz, 24 GB RAM, ATI Radeon HD 5700 Series

Motherboard
    Manufacturer    Dell Inc.
    Model    05DN3X (CPU 1)
    Version    00
    Chipset Vendor    Intel
    Chipset Model    X58
    Chipset Revision    13
    Southbridge Vendor    Intel
    Southbridge Model    82801JR (ICH10R)
    Southbridge Revision    00
    System Temperature    43 °C
        BIOS
            Brand    Dell Computer Corporation
            Version    A04
            Date    10/21/2010

Your system is infected with rootkit malware.

http://www.f-secure.com/weblog/archives/new_rootkit_en.pdf

The attack vector begins with an infected machine writing malware to an attached USB drive.
The malware program writes two driver files -- "mrxnet.sys" and "mrxcls.sys"
to the attached drive.  These rootkit files are very hard to remove.
The drivers serve "rootkit" functionality, disguising malware that is
subsequently written to the drive.

Clean reinstall will not remove the malware because the usb drive partions are infected as well. When the drives are inserted into a windows machine they REINFECT that machine. One way to fix this is to use a POWER PC Macintosh to remove the files to another drive then format the drives fat32 so that the rootkits are removed.  This is done by removing the partition and reformatting the entire drive with the OSX disk utility.

This can easily be proved by booting Hirens CD OR Live linux in which case all the drives will work as there is NOTHING WRONG with the hardware.

This is not a Dell issue and is not supported by Dell.

I appreciate your attempts to help. but it appears that this was not the problem.

I ran MalwareBytes mbar and Kaspersky Rootkit Killer and nothing was detected.

I did a clean install of Win7 Pro on a new drive and everything is fine with those USB drives. I believe the problem was a registry issue with my old installation. I suspected this because a drive I run via FW was also affected (could not load drivers).

If nothing else, this pushed me to install a Samsung 840 Pro SSD.

Thanks.