There should be no reason to add USB drivers to WinPE O/S unless there is something non-standard about your machine's USB ports. If you needed drivers for the base O/S install on the target machine then you would need them for WinPE as well, but that is rare unless you have USB-3 ports which do need drivers, hence our earlier questions.
What I am now puzzling over, and it may just be a matter of symantics, is the mention of trying to copy files to the X: drive. The X: drive is a virtual volume created by WinPE when it boots, and from which WinPE runs. It is certainly possible to write files to the X: drive once WinPE is booted, but they will have no permanence as the virtual drive is gone when winpe is terminated.
Re-reading the thread, I note that you are trying to read the USB device where you plan to store your source images, if I understand correctly. I assume you are planning to use 2Gb images if your USB key is formatted as FAT32, although you may have decided to format the key as NTFS and have single large images.
If you cannot find the USB key on any drive letter, then maybe no drive letter has been assigned to the USB device.
From the WinPE command prompt, type diskpart. At the diskpart command prompt, type lis dis
This will return the disk volumes that diskpart can see.
If the USB device is there you can assign a drive letter to it. Assuming it comes up as disk 3, type
sel dis 3
assign
This will assign a drive letter, at which point you can type Exit to return to the WinPE command prompt.
My article: https://www-secure.symantec.com/connect/articles/readyadventures-winpe will give you a more detailed description of working with WinPE.