I recently attempted to install Ubuntu 12.04 LTS Server from a USB and encountered a weird problem. After downloading the Ubuntu ISO image, I utilized the Universal USB Installer from pendrive.com to make my USB stick bootable.
The USB stick booted fine and I began the installation process. However, very quickly, the installer presented me with the following error message,
Your installation CD-couldn't be mounted. This probably means that the CD-ROM was not in the drive. If so you can insert it and try again
Retry mounting the CD-ROM?
I’m not sure what causes the above error, but, here is how to overcome this obstacle.
Copy the ISO image to the USB stick and attempt to install Ubuntu again. When the above error message appears, select “No” and then issue the following commands:
mount -t vfat /dev/sdb /media/usb
mount -t iso9660 -o loop /media/usb/ubuntu-12.04-server-amd64.iso /cdrom
Where /dev/sdb is the device for your USB stick. You may have to explorer other devices. Try typing ls /dev/sd* to see the available drives if you encounter troubles here. The idea is that we’ll manually mount the CD-ROM drive ourselves and then let the installer resume as if nothing out of the ordinary has happened. So, after the ALT+F1, simply select,
Load debconf preconfigruation file
from the menu and the installation will resume. Good luck.