First my compliments for this post:
Installing Ubuntu Alongside a Pre-Installed Windows with UEFI
It explains the dual boot installation very well. The post also says that installing on a Lenovo laptop is NOT possible. However, I doubt this. I would like you to present you all details and show you how far I got and where i'm stuck.
Dual boot on a Lenovo B50-70 laptop. OEM version of Win8.1 64 bit. Bios UEFI mode on, Secure Boot on. All preparations done like partitioning and Win shut-down without fast start up. LiveUSB with Ubuntu 14.04.1 64 bit, created with Unetbootin. I use a USB 2.0 connector, not a USB 3.0.
My first attempt failed because Ubuntu was installed in BIOS mode. The BIOS setting for UEFI was off. I used boot-repair but that didn't work so I used Lenovo's recovery to restore everything and Windows booted again.
I turned on UEFI and tried again.
In my second attempt Widows started right away ignoring the LiveUSB. This was because of the boot order. In BIOS the Win and USB device was visible. Setting the boot priority or directly by the boot menu made the GRUB bootloder start.
On startup I got a GRUB menu
Grub had just one entry which is a little confusing: "Boot-Repair-Disk session". I used the USB stick previously for Boot-Repair but then wrote the Ubuntu iso to it. Anyway when I select it, Ubuntu booted.
The dark purple Ubuntu screen showed and made me think every thing would be ok, but after some time the purple screen is replaced by:
BusyBox V1.21.1 (Ubuntu 1:1.21.0-1ubuntu1) built-in shell (ash)
(initramfs) unable to find a medium containing a live file system
At this point I'm stuck. If I check where I am, I can see a file system with dev,root,var,inti etc. etc.
There are 2 filesystems: udev and tmpfs.
The file casper.log shows the same error as shown above.
Is there a way by entering commands in the Grub menu or in the BusyBox shell that I can start the LiveUSB?
My first guess was that Secure boot generally prevents booting of any other OS and also the LiveUSB. However, turning Secure boot off in BIOS has NO effect....
I am open to all suggestions. Even if GRUB must be started by the Windows bootloader.