Install Ubuntu from Alternate CD image; booting from USB flash drive

I recently tried a couple of ubuntu versions, installing them on a partition to play a few hours. In the process I re-discovered unetbootin, a wonderful tool for this: I burned the CD ISO images in a USB flash drive, instead of burning CDs (even if you use rewritable, it is slower than a USB key). UNetbootin let's you make a bootable pendrive from any ISO image; you can even download your favorite distro CD from there.

The main comment of this post is, however, not just the unetbootin process, but two tips that solved issues with these images.

First, the simpler one: You have to mark the partition in the USB drive as bootable, or you will not be able to boot... To do this, run fdisk /dev/sdb (or whichever your drive's device is), and add its boot flag.

The second on is subtler. With the Ubuntu 9.10 Desktop CD everything worked perfectly; I was able to boot. But later I tried the Alternate CD, which after booting it could not find the contents of the "CD" it was installing from. Somehow it missed the point it had boot from USB; so there would not be any contents on CD-ROM to read (in fact, that machine does not even have a CD drive). What I had to do was to add a flag to the kernel boot options: When UNetbootin starts, before pressing Enter on the "Default" option, press Tab to edit it. Edit the kernel line, to make it look like this one:

/ubnkern initrd=/ubninit vga=normal cdrom-detect/try-usb=true -- quiet

(Everything should be in a single line). Your line might not be identical; but the important part is to add "cdrom-detect/try-usb=true" before the "--". With this option the installer found the contents and was able to continue the rest of the install process.