Hi Tom,
From your previous posts, it appears that you have a Tyros5, which although has quite a small USER drive (approx. 6.7 MB), has a huge 500 GB Hard disk drive! I personally would use this as my main storage drive, although it is not quite as quick and easy to back it up to a computer as a much smaller USB! The OP was concerned about using a USB in case he forgot to take it along when playing out of home, but in your case you can't easily forget to take a built in hard drive with you!
However, for people with keyboards such as PSRs, that do not have an internal hard drive or large USER drive, what I was suggesting was to use a USB drive as the main storage drive for your keyboard, and reserve the keyboard's relatively small USER drive for essential system files and temporary work space, which you can 'clear out' every now and then.
Then, as you say, you would access the files directly from the USB. File access time from USB would be very slightly slower than from USER, but this should not normally be apparent when using the keyboard, unless you are using an exceptionally slow USB device, or it is faulty.
You can structure the folders on the USB any way that you want, and copy everything from the keyboard's USER drive that you can, but I would not recommend creating a folder named 'USER' since this is likely to become very confusing!
Of course, if you do not intend to save many of your own files, for example edited styles, voices, registrations, multi pads etc., then your existing USER drive may be large enough for your needs, and it would hardly be worth the bother to switch to using a USB, depending on which keyboard model you have, and the size of its USER drive.
Remember too that any existing registrations that use styles, multi pads etc. that are currently saved to the USER drive, would need to be modified if you move these associated files. The same applies to any of your own Music Finder records that call up styles or songs currently saved on the USER drive. This may be another reason why someone decides to continue to use the existing set up if space is not an issue for you.
However you need to think ahead, and if you think that it is possible you will fill the USER drive in the future, then the earlier that you switch to using a USB (or HDD) as your main storage drive, the easier the process is likely to be.
Regards
Ian