This happens a lot when trying to install Ubuntu Server 14.04 from USB stick (14.04.3 still has the issue).
[!! Detect and mount CD-ROM]
Your installation CD-ROM 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?
.. and of course there is no CD-ROM at all, the installation media are on the USB drive. An easy way to work around this:
- Open a second console with Alt-F2 (get back to the installation dialogue with Alt-F1 later)
- Press enter to activate it
- Enter the following command to make /cdrom a softlink to /media:
rmdir /cdrom; ln -s media /cdrom
- Go back to the installation process with Alt-F1
- Answer ‘yes’ to the question Retry mounting the CD-ROM?
From here on the installation should go on as normal.