I installed Linux on a flash drive and after booting my computer, I encountered an error:
GRUB Loading Stage 1.5.
GRUB loading, please wait...
Error 21
From what I read, this means that I can no longer boot windows normally without having the flash drive plugged in because my Windows MBR was corrupted by the installation of GRUB Stage 1. Anyway, a great explanation of this mess can be read here (start at the second paragraph).
MBR can be easily repaired with the Windows recovery disc, which I don't have but I did find an easy solution which worked well for me.
ADS HERE !!!