[ubuntu] Grub Does Not Recognize USB Flash Stick During Boot

Computer: HP Envy Intel I7, SSD, Ubuntu 14.04 LTS, EFI in (hd0,gpt1), system installed in (hd0, gpt2).

USB stick: GPT-partitioned, EFI in (hd1,gpt1, with grub-boot flag) and the same Ubuntu 14.04 LTS system installed in (hd1,gpt2).

I did an update-grub such that I can see the option of booting into the USB stick during grub boot. However, after I selected this boot-to-(hd1,gpt2) option, an error message was shown, basically saying that (hd1,gpt2) was not found. Initially I used the EFI flag for the (hd1,gpt1) partition, same result.

I went into the grub prompt and entered an "ls" command. The output only shows the partitions on the (hd0) drive, no (hd1) partition was recognized.

After I booted into (hd0, gpt2), I can see all the partitions on the flash stick (hd1). However, again as stated above, the flash stick (hd1) was not recognized during grub boot.