There is software that will overwrite data on the hard drive. Personally, I would dismantle the laptop, remove the drives, strip the drives and smash the platters. There is little value in selling old computers.
Mac laptops may likely have solid state drives (not hard drives). Those drives are 2.5" form factor and can be replaced by hard drives.
Laptops use 2.5 inch hard drives, while desktops use 3.5 inch hard drives.
One can find more information about external hard drives for laptops from Amazon. The best way to find more information is from the brand of laptop website.
Hitachi, Seagate, Maxtor
Yes there is a way.
Hard drives now are pretty robust. You should try not to move the laptop when it is on but it really should not hurt the hard drive.
Any that are USB.
people are buying laptops because laptops hard drives have maximum capacity they can easily upgraded . and specially they are more portable
Encrypt the contents and either wipe or destroy them when you dispose of them
Yes. SSDs (Solid State Drives) are still much more expensive per gigabyte than magnetic hard drives, so they are still uncommon in desktops and mainstream laptops.
Not all portable hard drives work with every laptop. Some portable hard drives are formated for specific types of laptops. For example a portable hard drive that is formated for a Mac cannot be used with a windows laptop
Yes. Laptops use 2.5inch drives.