The major product when 3-methylbutan-2-ol is treated with concentrated HBr is

The secondary alcohol is protonated and loses water giving the secondary carbocation $\mathrm{(CH_3)_2CH{-}C^+H{-}CH_3}$. A 1,2-hydride shift moves the cation to the adjacent tertiary carbon, $\mathrm{(CH_3)_2C^+{-}CH_2CH_3}$, which is more stable. Attack by $\mathrm{Br^-}$ gives 2-bromo-2-methylbutane as the major product, and 2-bromo-3-methylbutane (direct substitution) is minor.