Does safely ejecting from a USB port actually do anything?Phillip Remaker^[1]:Is there any harm to be incurred by just pulling a flash drive out? Why do we need safe removal at all?

Historically, operating systems (OS) treat disks as objects that can be trusted not to change state suddenly. When reading or writing files, the OS expects the files to remain accessible and not suddenly disappear in mid-read or mid-write. If a file is open, a program reading the file expects to be able to return to it and continue reading. Similarly, write commands may be dispatched to a writing subroutine and forgotten by the main program. If a drive disappears between the time the subroutine is called and the data is written to disk, that data is lost forever.

In ye olde days, there were formal processes to physically “mount” and “unmount” storage media, and the physical act of mounting a tape or a disk pack triggered some mechanical switch to detect the presence or absence of media. Once the mechanism was engaged, the software could start to use the media (a “soft mount”). Some media even had mechanical interlock to prevent media from being ejected or removed until the software processes using the media released the lock.

The Macintosh floppy and optical disk provide more modern examples of an interlocked physical and soft mount. One could only eject media through a software command, but that command might fail if some program was holding a file open on the medium. 

Enter USB connected storage. There is no mechanical interlock in a USB connection to coordinate the hard and soft mount. The user can decide to rip the disk out from under the operating system at any time, and endure all manner of programs freaking out about the sudden loss of media. “Hey! I was using that!” Symptoms could include: Lost data, corrupted file systems, crashing programs, or hanging computers requiring a reboot. A safe removal executes the “soft unmount” needed to prevent any unexpected Bad Things that may happen if a program loses its access to media.