Swanto,
Drift is a special form of movement that happens AFTER all other moves have been made. So, if you anticipate a hulk will drift, then realize it will drift AFTER your moves are made. Therefore, the drifting hulk WILL NOT vacate the hex it is in prior to your moving, and you will suffer a collision and possibly a foul. (Fouling occurs 50% on any collision). Unfouling occurs 33%, but both ships may attempt it each turn.
Take Care,

Derek