Halfway through making a Linux USB, Rufus pauses and asks a question most guides never explain - ISO mode or DD mode. The two options produce completely different drives. Here is what each one does, in plain language, so the choice takes five seconds next time.
The dialog appears because some images are ISOHybrid - built to work both as a disc image and as a raw disk image. Rufus can write such an image two ways. ISO image mode opens the image, extracts its files onto a normally formatted drive, and sets up booting. DD image mode clones the image onto the drive byte for byte, exactly as its author built it, without interpreting anything. Same ISO, two very different results - and the right answer depends on what you need the drive to do afterwards.
In ISO mode the finished drive is a readable, ordinary file system. You can open it in Explorer, see the files, even add a folder of drivers next to them. For supported Linux distributions this mode is also what makes the persistence option possible - a reserved area where the live system saves changes between reboots. Rufus may download a small boot file or two (for example a newer Syslinux version) to make the image boot correctly; that is normal. Windows installer images are always written this way - extraction is how a Windows USB is built.
DD mode is a photocopy, not a conversion. Every byte of the image lands on the drive in the author's exact layout, which makes it the fallback for images whose boot structure ISO mode disturbs. The costs are real: afterwards Windows shows the drive as tiny or unreadable - often a few megabytes - because the partition layout is the image's own, not a normal one. You cannot browse or add files, persistence is not available, and the missing capacity only comes back when the drive is reformatted or its partition table is rewritten.
The shrunken, unreadable drive alarms people into thinking DD mode broke the hardware. It did not - the drive simply carries a foreign partition layout. To reclaim it: run Rufus again and create a non-bootable drive, or open Disk Management and delete the partitions, then create one fresh volume. Either takes a minute. The same warning in reverse applies to the Ventoy comparison: this whole question exists only because Rufus writes images directly. On a Ventoy drive the ISO is never extracted or cloned at all - it is booted as a file - so there is no mode to choose, though occasionally an image that needs a true DD-style write is exactly the kind that still boots better from Rufus.
Watch the dialog appear in the video and see both finished drives compared - the browsable ISO-mode drive and the strange tiny partition DD mode leaves behind.
Watch Full VideoISO image mode - always. Windows installer drives are built by extracting the image and adding the correct boot files, and Rufus handles Windows images in this mode by design. The DD question mainly concerns hybrid Linux and utility images.
No. Windows is showing you the small boot partition of the cloned image and cannot read the layout as one normal volume. Delete the partitions in Disk Management or rewrite the drive with Rufus as non-bootable, and the full capacity returns.
No - the mode is decided at write time and the two results are physically different layouts. Switching means writing again. The good news is that a rewrite takes only a few minutes, which is why trying ISO mode first costs almost nothing.