Problem installing 12.04 in efi mode

I don't know why it failed to mount the filesystem. Before you proceed, though, you may want to read my Web page on EFI boot loaders for Linux. It contains a lot of background information you may find helpful. Also, I recently heard that Ubuntu 12.04.2 has been released, and it includes some improvements to its EFI support. Thus, if you're not already using that version, you might want to give it a try.

Beyond that, here's what I suggest:

1. Before installing, use a Linux emergency disk to pre-partition the disks. Use a GUID Partition Table (GPT) scheme, create an EFI System Partition (ESP) on at least one disk, and create a separate non-RAID Linux /boot partition that uses ext2/3/4fs or ReiserFS. The former is required to boot an EFI-based computer. The latter isn't required by all boot loaders, but if you don't have such a partition, you'll be limited to using GRUB 2, which doesn't always work well. With a separate /boot partition, you'll be able to choose from among several boot loaders.
2. When you boot the installer, drop to a shell and look for the directory /sys/firmware/efi. If it's not present, you've probably booted in BIOS mode, which won't work -- or at best, it will require jumping through some extra hoops after installation. Thus, if this directory is missing, reboot and try to find your firmware option to select the boot mode (BIOS/legacy/CSM vs. EFI/UEFI).
3. If you still see the error that you report or if you can only boot in BIOS mode, you should be able to complete the installation and then install your boot loader or boot manager from an emergency system. This will require extra effort, but it can be done. See the page I referenced earlier on EFI boot loaders for details. If you can't figure out how to boot the installer in EFI mode, though, it's conceivable that your computer is BIOS-only or has such a buggy or restrictive EFI that it's not worth the effort to try to force an EFI-mode boot.

I reformatted my main partition with ext4:

1. Went into terminal with Ctrl+Alt+F1.

2. Then formatted the partition as super-user:

   sudo mkfs -t ext4 /dev/sda2