You don't "get" it. You create one from a directory containing the files you want to be in the file system image. A SquashFS image can be created by using the command

mksquashfs /path/to/dir imagename.sqsh

True read-only USB disks require hardware support. Otherwise, you can still erase the disk with the same tools you used to create it in the first place.

One method to create a read-only file system, that should work on most Linux distributions, is to use SquashFS.

1. Create a directory (such as squash) and put the files you want on the disk in it.
2. Create a SquashFS image of your directory. mksquashfs ./squash squash.img
3. Partition the flash drive (such as with cfdisk or GParted). Make the partition the same size as or slightly larger than your SquashFS image.
4. Write the SquashFS image to the partition you created. dd if=squash.img of=/dev/sdb1 bs=512.

Several, such as ext2/3/4, ReiserFS, btrfs, cramfs, romfs, SquashFS, and Xiafs.

It is very difficult to build a LiveCD from scratch. Basically, you need a kernel that can read ISO9660 images, a file system image (either squashfs or cloop), and an overlay system, to make it look like you are changing things on the file system, but are erased on shutdown. The overlay is one of the most difficult things to construct from scratch, which is why I recommend you use a base CD from an existing LiveCD distro.

There are several distros you can use as a base, and several tools you can use to help you. Ubuntu, Debian, Knoppix, and Slax are the most popular bases for LiveCDs. To remaster the image, you can:

* Use Remastersys (for Ubuntu). * Use DRUDell (for Ubuntu) * Use Revisor (Fedora) * Use mklivecd (Debian and Madriva) * Use Custom NimbleX (online LiveCD creator, based on Slackware) * Do it manually, via the command line.

The instructions vary per distro on the command line. Basically, you copy all the contents off an ISO image, mount the compressed file system, copy it's contents to another folder, chroot into those contents, make changes, build a new compressed image, and build the contents back into an ISO image. While this method is more difficult (or at least more tedious), it allows you far more power to customize.