Comment 9 for bug 557388

And I believe I now know what the scenario which is leading to this error is. UNetbootin has an option to put the live environment (squashfs and all) onto a hard drive partition (the hard drive install mode), so that it can be booted in live mode. If a user has a single system partition, puts the live Ubuntu environment onto that partition, and attempts to boot it, it will boot into live mode correctly.

However, if the user launches the installer, then the only partition which can be installed to is the same hard drive partition that the live Ubuntu environment is running from. Hence, when the partitioner starts, it will attempt to unmount that partition (which is currently mounted to /cdrom), and this will of course fail because that's where filesystem.casper et al reside. Hence the user gets the "Can't unmount /cdrom" error. The user now manually unmounts /cdrom, starts the installer, and gets this error.

So, I believe the solution would be to display an error message more along the lines of "you can't install to the partition that you booted the Ubuntu live environment from; please use a CDROM or USB drive to install".