Ubuntu – Grub loads instead of Ubuntu whenever the HP streambook reboots

 => Syslinux MBR (4.04 and higher) is installed in the MBR of /dev/sda.
 => Syslinux MBR (4.04 and higher) is installed in the MBR of /dev/sdb.

sda1: __________________________________________________________________________

    File system:       ntfs
    Boot sector type:  Windows 7/2008: NTFS
    Boot sector info:  No errors found in the Boot Parameter Block.
    Operating System:  
    Boot files:        /bootmgr /boot/bcd

sda2: __________________________________________________________________________

    File system:       ntfs
    Boot sector type:  Windows 7/2008: NTFS
    Boot sector info:  No errors found in the Boot Parameter Block.
    Operating System:  
    Boot files:        /bootmgr /Boot/BCD

sda3: __________________________________________________________________________

    File system:       ntfs
    Boot sector type:  Windows Vista: NTFS
    Boot sector info:  No errors found in the Boot Parameter Block.
    Operating System:  Windows 7
    Boot files:        /bootmgr /Windows/System32/winload.exe

sda4: __________________________________________________________________________

    File system:       Extended Partition
    Boot sector type:  -
    Boot sector info: 

sda5: __________________________________________________________________________

    File system:       ntfs
    Boot sector type:  Windows 7/2008: NTFS
    Boot sector info:  According to the info in the boot sector, sda5 starts 
                       at sector 2048.
    Operating System:  
    Boot files:        

sdb1: __________________________________________________________________________

    File system:       vfat
    Boot sector type:  SYSLINUX 4.07 2013-07-25
    Boot sector info:  Syslinux looks at sector 58112 of /dev/sdb1 for its 
                       second stage. SYSLINUX is installed in the /uui 
                       directory. No errors found in the Boot Parameter Block.
    Operating System:  
    Boot files:        /boot/grub/grub.cfg /casper/vmlinuz.efi.signed 
                       /EFI/BOOT/grubx64.efi

The boot-repair log clearly shows that there is no sign of installed Ubuntu on your HDD. Maybe your Ubuntu partition got formatted or something else. So please try to reinstall Ubuntu on a separate partition wherever you want.

https://help.ubuntu.com/community/WindowsDualBoot

Unfortunately, the instructions were for a self-installed copy of Windows 8, so their setup was NOT a UEFI machine. Things are a little different for preinstalled Windows 8 on UEFI machines.

External (removable) boot devices should have their own 300M, FAT, EFI partition to hold the bootloaders. When you select the location for the bootloader (on sdb for the external), use this partition (which may work) instead of the device (which never works). When it doesn't work, the internal hard disk is used, so just copy the files from the internal disk's EFI to the external disk's EFI. Now, the installer still may not set up the bootloaders into the right directory for a removable device -- they should go into /EFI/Boot, and be named /EFI/Boot/bootx64.efi. Instead, grubx64.efi and shimx64.efi (the bootloaders) typically get put into /EFI/ubuntu (just like on the internal hard disk). I like to use shimx64.efi as the bootx64.efi file so the boot works whether secure boot is enabled or disabled. So, copy /EFI/ubuntu/shimx64.efi to /EFI/Boot/bootx64.efi (note the name change), and also copy /EFI/ubuntu/grubx64.efi to /EFI/Boot/grubx64.efi. The grub.cfg file may be left in /EFI/ubuntu.

The nvram boot entry for USB (disks...) should now boot the external disk -- no new entry in nvram should be needed. If you are getting grub without the external hard disk, looks like it got installed to the internal hard disk's EFI (no problem), and was put before the Windows bootloaders in boot order (problem). efibootmgr from Ubuntu will allow you to change the boot order, or delete the unwanted Ubuntu entry (remember, you will be booting with the USB entry).