See if you have file /proc/config.gz in your stock kernel. You can gunzip it and use it as a base from which to start configuring the kernel. It's an ordinary kernel configuration file which you can load from within kernel configuration utility. Do not remove any options that might have something to do with usb mass storage.
At least you need scsi disk support and usb mass storage support. Linux-Googling (
www.google.com/linux) with "usb mass storage" gave lots of results, check them out if you still run into trouble.