All modern operating systems write hidden and sometimes not-so-hidden data to hard drives as soon as the drive is plugged into a host computer and powered on. This behavior upon disk mount is done automatically and typically without any user consent. Hidden folders like "Recycle Bin" (Windows) and "Trashes" (Mac OS X) are created/updated automatically. Spotlight or Windows search database indexes and graphical icon databases are created/updated. The "last touched" or "last accessed" timestamps on files and folders are updated automatically when merely browsing the drive. There's actually quite a bit of behind-the-scenes stuff that operating systems perform on data that they manipulate on every connected hard drive, regardless of how many settings are tweaked trying to turn those "features" off. Although most of this operating system cruft is presumably well-intentioned, it can cause compatibility problems when shuttling the drive between machines.
Lurking beyond the operating system mechanics are more insidious threats to hard drive data integrity. An undetected virus residing on the host machine can wreak havoc on client drives, writing copies of itself on every connected drive and corrupting data along the way. There's also the ever-present likelihood of "user error" — accidentally deleting the wrong file or formatting the wrong drive. No matter how carefully one treads when juggling multiple hard drives, dragging-and-dropping files or hammering away at the command line, Murphy's Law eventually and gloriously prevails.
