If you're dual-booting with Windows 8 or 8.1, you must disable the Fast Startup feature in Windows, as described here, among many other places. This feature turns a shutdown into a suspend-to-disk (hibernate), which leaves filesystems in an inconsistent state. This can result in an inability to mount the filesystem(s) in question from Linux, although I don't recall offhand if the "IO charset iso8859-1" error message is a possible symptom in Linux. (I suspect not, offhand, although I'm not positive of that.) Note that the Windows Fast Startup feature is not the same as a feature with the same or similar name in many EFIs, so disabling such a feature in your firmware is not adequate (or even necessary) to fix this issue.
In the short term (for a single boot), the EFI System Partition (ESP), which is mounted at /boot/efi, need not be mounted for most day-to-day activities. Thus, pressing S to skip mounting it should get Ubuntu to boot. This should enable more diagnostics and repair attempts. I recommend:
- Once you've booted, try mounting the filesystem manually, as in
sudo mount /dev/sda1 /boot/efi. This may produce a more informative error message; or you can type dmesg | tail after a mount attempt to look for anything that might be recorded to the kernel ring buffer. Note that if you do get the partition to mount, you can unmount it with sudo umount /boot/efi if you want to try again (say, to try various options to figure out what's working and what's not working).
- The
/etc/fstab file contains entries that describe what to mount where and how to mount each filesystem. Try locating the entry for /boot/efi and do a Web search to learn more about this file. This won't help you per se, but you may discover a mount option that will fix the problem, in which case editing the /etc/fstab file may be in order.
- In a worst-case scenario, backing up the ESP, umounting it, creating a fresh filesystem on it (via
sudo mkdosfs -F 32 -n ESP /dev/sda1, re-mounting it, and restoring it may be necessary. A file-level backup (via cp, tar, or the like) should be adequate; but you may need to adjust the /etc/fstab file because the serial number (UUID= in /etc/fstab) will almost certainly change. Of course, you'll need to successfully mount the ESP in order to back it up.
- It's conceivable that your kernel has been built incorrectly; or more likely, that your initial RAM disk (initrd) has been built incorrectly. This type of problem is difficult for most users to overcome, although Ubuntu's
update-initramfs tool can help with the latter.
