If you are dismantling a hard drive you will find a lot of Torx (* slotted) screws and they will generally come loose pretty easily if you pick the correct Torx driver. I have no trouble usually finding one (the shop I work in probably has a dozen or more sets of these); if you are too big, or too small, you will have trouble with getting them free and maybe will strip the head or slip a lot. Also watch out for rounded drivers (usually used in cars and such) as they won't take up properly and you will get no where with them.
Some of the Torx screws (also called "security" Torx) have a raised part in the middle and need this kind of driver:
This "security Torx" screw looks like head on:
You can use a "security" Torx for non-security screws but not the other way around. You can also use the "right" size of hex driver but you'll end up ruining the tool or stripping out the screw sooner or later!
The best thing you can have is a nearby friend with lots of tools! I have a friend who might as well be named MacGyver (MacGruber?) because of all the tools he carries around in his car trunk and pockets!