An "ISO" is nothing more than a compiled authoring session for optical disc. Using the term ISO and the meaning "compiled authoring session", it is not a stretch to have expected you to understand what was meant. Or maybe it was. Then, you CRY about making the file format of the compiled session that of a specific file system type, which if you did not merely cut and paste this app together, you would have fairly deep knowledge of as well. You also have your app extracting a boot sector from a DOS file system disk, yet you have no problem writing that extracted file out. The request to have your application, which is essentially a tool for putting together an authoring session, write out to other than an optical device is not that much to ask or even want to achieve.
It appeared from the answer to the other person that asked that you think that the write session to a USB is where the problem would occur and that you would have to translate the workings of hundreds of brands of flash drive to your application. I do not think that there is a problem, since any inserted flash drive gets recognized by the OS, and gets mounted already. And finally, yes, it WILL become more and more popular. Particularly when more and more computers are being assembled without a floppy drive and with USB booting capacity. It pretty much goes without saying. But hey... if you want to remain a half decade behind the rest of the world, that is your gig. I am sure that other authoring tools will emerge where the author is not so stubborn as to be thought of as being bone headed.
As far as existing tools go, they ALL ONLY allow a SINGLE partition to be made on the stick. I want a boot partition, and then additional partitions. No, I know that you did not make a partitioning tool, you made an authoring tool, but we are not talking about partitioning a hard drive, we are talking about prepping and using a USB drive, and I thought that my suggestion to you would vault you to the top of the heap in user tools. The choice is, of course, yours. But if you think there is no demand, then why not simply run the poll, or do a few quick google hunts that would show you that there are hundreds of folks out there manipulating their flash drives. A USB drive is not 'monitored' the same way standard hard drives are, and besides, your app could easily gain full access to any USB drive as they have about the same level of 'tamper proofing' that a floppy drive does, and I know of no sectors that are unavailable except on a drive that someone shouldn't be using for the purpose anyway, such as encrypted drives that have proprietary code segments on them and proprietary drivers on the host machine as well. It really doesn't matter as you are going to do whatever you want anyway, and the enticement of having the ultimate tool doesn't motivate someone who is fat, dumb, and happy with what he has. Thanks anyway.