Do all computers contain a removable hard drive?

From the point of view of construction, yes. At some point the hard drive had to be put in the machine and integrated into the system. It's a separate component of computers. Whether or not that means it can be removed and replaced is another matter, and the techie who looks at your machine may or may not be able to R & R the unit. Certainly as a user you'll have to weigh your skills and abilities against the difficulties associated with removing the drive yourself. If the consideration is one based on "portability" of data, grab an "add-on" or external hard drive to store data you need to truck from point A to point B. It only takes a small application (software package) on a given machine to get the external drive to work with that PC. These portable units are getting cheaper and more robust every year - or every 18 months, if you believe Moore's law (as it's stretched to apply to hard drive capacity as opposed to semiconductor density, which it originally addressed). The "remove-ability" of the hard drive says nothing at all about software considerations. Far and away the most common residence of the operating system (OS) in a computer is the hard drive. No hard drive, no OS - and there's no way your machine can come up and run.