When you burn a bootable USB can you then use the rest of the USB stick for storage? Say put a storage folder beside the files needed to boot and put other data files in the storage folder.
Absolutely... BUT, you must remember, if you "re-use" your BOOTable USB for the BOOT section update, the update process may erase the UFD before re-creation of the BOOTable section. And there are other issues if you decide to multi-partition that UFD to separate that DATA (Windows versions, etc.) Go cautiously...
I think it depends. I have seen Linux boot filesystem that were exactly the size of files inside od them so you could not write anything new without altering/corrupting files. I also saw much bigger filesystems created by tools that haf a lot of free space. I also saw multi partition schemes, where EFI bootloader was tightly packed but rest had some additional space. In general UEFI, with new bootloader and GPT gives more flexibility.
