The title here should be "How to get rid of NOOBS". The whole trouble is described here viewtopic.php?f=91&t=213850
. The source system is NOOBS 2.4.1 with Raspbian and the poster wants to create a bootable USB stick from this source.
I never did such a conversion. Everything what I write is a guess.
Downloadable Raspbian images have 2 partitions. The first fat32 "boot" partition is mounted on /boot and the second ext4 "root" partition is mounted on /. Raspbian maintains these two partitions when you run
so everything, you need to create a bootable USB stick should be in these locations.
Plug your SD card and your USB stick into your Raspberry Pi. Create the "boot" and "root" partitions on your USB stick if not already present. Format these partitions. Copy all files below /boot to the "boot" partition of your USB stick. Copy all other directories of your SD card to the "root" partition of your USB stick (don't copy the contents of your mountpoint for the USB root partition). Then edit cmdline.txt and etc/fstab on the partitions of your USB stick.
I can't promise, that this works. I used NOOBS a few days only. But if I needed to migrate from an outdated NOOBS with Raspbian to "clean" Raspian, this would be the second choice, what I would try, just for curiousity. My first choice would be a new installation.