I had the same question once. Why can't I just load the aft external reserves to the internal bow reserves?
As it turns out, the external reserves are places in a way so that they can quickly be placed into the U-Boat directly.

Im sure I can get a better picture.
Now imagine taking out aft or bow reserves and taking them to thier opposite internals, in port this is no problem with cranes and stuff, but out at sea, your crew better becareful not to drop it into the water.
I hope that was the answer you were expecting.
Oh and hello, hiram and welcome aboard Herr Kaleun

( it's always polite to welcome new comers)