So you can't usually take the boot loader from the ISO (which would usually be an El Torito boot loader), copy it to USB, and expect that USB to boot.Īnd this is the part that is relevant to our list options, because Rufus will have to provide a relevant bootloader piece, that it simply can't obtain from the ISO. Unfortunately, HDD/USB and ISO bootloaders are very different beasts, and the BIOS or UEFI firmware also treat USB and optical media very differently during boot. the first bit of code that executes when a computer boots from USB. Then, there is the actual bootloader itself, i.e. But of course that means that, whoever create the ISO system must have taken some provision to support FAT32 or NTFS as a file system for live or installation (which not all people, especially the ones who rely a bit too much on ISOHybrid, tend to do). Most of the time, this means extracting and copying all the files from the ISO onto a FAT32 or NTFS file system, which is what bootable USB Flash drives tend to use. One of the first thing that usually needs to go is the ISO9660 or UDF file system that ISOs used. How does that relate to the list options? We're coming to that. But that also means that some conversion must (usually) occur, so that our round ISO peg can fit nicely into the smaller square hole that is an USB flash drive. This is why OS providers tend to want to stick with ISOs where possible, as it (usually) provides a better user experience across all OSes. For instance, this will be the case if you use the FreeBSD USB images on Windows - Once the USB has been created, Windows will be unable to access any content on it, until you reformat. Also, whereas optical discs, and therefore ISOs, can only ever use one of two file systems ( ISO9660 or UDF), all of which have been very well supported in all major OSes for a very long time (which allows you to take a peek at the image content before or after you use it), DD Images can literally use any of the thousands of different file systems that exist, which means that, even after you create your bootable USB, you may not be able to actually see any content on it until you boot. Well, outside of historical reasons, one of the issues of DD Images is that, because they are a partitioned file system, then if you create a 1:1 copy on USB media, and your media is larger than the one from the person who created the image, then, then you will end up with the apparent capacity of your media reduced to the size of the one from the original DD Image. Now you may be wondering why, if bootable ISOs are so poorly suited for that, most OS distributors out there provide ISO Images instead of DD Images. Thus, we have established that ISO images are actually poorly suited to create bootable USB media, because they are the equivalent of providing a round peg in order to fit a smaller square hole, and therefore, the round peg must be altered in order to fit. For the record, in Rufus terminology, a 1:1 copy of USB media is called a DD Image (you'll see that option in the list), and some distributions, like FreeBSD or Raspbian, actually provide DD Images for USB installation, alongside ISOs for CD/DVD installation. That's not to say that this kind of 1:1 copy cannot exist for USB, just that, 1:1 copies of USB media will be completely different from 1:1 copies of optical disc media and (outside of using ISOHybrid images, that are crafted to work as both 1:1 copies of disk and optical) are therefore not interchangeable. what CD/DVD burner applications do when writing an ISO). So, if you have an ISO, you cannot simply do with an USB what you can do for an optical disc, which is read every single byte from the ISO and copy it as is, in sequence, onto the disc (i.e. ISO is a 1:1 copy of an optical disc, and optical disc media are very different from USB media, both in terms of how their boot loaders should be structured, what file system they use, how they are partitioned (they aren't), and so on. To start with, you have to understand that the ISO format was NEVER designed for USB boot. when the Advanced Options section is displayed), because they are intended for people who already known what they are about. First of all, a lot of the options you list are only be listed when running Rufus in advanced mode (i.e.
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