Setting up lenovo yoga 900 with ubuntu is not straight forward.
Get into bios by pressing the tiny circular button on the right side of the laptop with a toothpick when the computer is off.
- Enable back update in bios
- Boot to windows, dowload linux-specific bios update from lenovo, install it and follow instructions.
- Change configuration in bios under "configuration" from RAID to ACHI.
- Boot from your ubuntu usb-stick, pressing fn-f12 during restart.