In case you are wondering why all of the sudden your OS doesn't automatically mount your flash drive and you had to manually assign drive letter, this may caused by your auto mount is disabled. Usually, people who recently installed disk/partition management software will encounter this problem.
To rectify this, simple re-enable the automount.
1. Launch command prompt: CTRL+R > type 'cmd' > enter
2. Run diskpart utility: type 'diskpart' > enter
3. Enable auto mount: type 'automount enable'
Hope this helps :)