Disk images explained: ISO, IMG, floppies and kernels
What the file you are about to boot actually is, which slot it belongs in, and why picking wrong gives you a blinking cursor.
A disk image is a file containing the exact bytes a disk would contain — not a copy of the files on the disk, but a copy of the disk. That distinction is the whole reason images work: the guest reads sector 0 and finds what it expects there, including the parts no filesystem would ever show you.
The four kinds you will meet
ISO — a CD or DVD
The most common form. An .iso is a read-only optical disc, usually carrying an El Torito boot record that the BIOS knows how to start. Live distributions ship this way because a CD cannot be written to, which forces the system to be self-contained. Mount it as CD-ROM and put cdrom first in the boot order.
IMG — a floppy or a hard disk
An .img is just raw sectors, so the extension tells you nothing about which it is. Size does: 1,474,560 bytes exactly is a 1.44 MB floppy, and anything substantially larger is a hard disk. Get this wrong and nothing boots — the floppy controller and the IDE controller are different devices, and a hard disk plugged into the floppy slot is not readable.
bzImage — a kernel with no disk at all
A compressed Linux kernel, booted directly. There is no BIOS boot sector and no bootloader; the emulator loads the kernel into memory and jumps to it, passing a command line. This is the shortest path from power-on to running code, which makes it excellent for seeing what a kernel does before userland exists. Buildroot and NodeOS both boot this way.
initrd — a filesystem in memory
A small archive the kernel unpacks into RAM and uses as its first root filesystem. It pairs with a bzImage: the kernel needs *something* to run as process one, and an initrd is that something when there is no disk.
Boot order
The BIOS tries each device in turn and boots the first one that has a valid boot signature. If your image is a CD but hda comes first and there is an empty blank disk attached, the BIOS finds nothing bootable on the disk, moves on, and everything still works. If the order puts a *bootable* device you did not intend first, you will boot the wrong thing. When a machine hangs on a blinking cursor, boot order is the first thing to check.
Blank disks
You can also attach an empty disk of a chosen size — useful when you want a system to have somewhere to install itself, or somewhere to write. It is zeroed, it has no partition table, and like everything else here it disappears when you reload.