Here's the actual explanation for a five year old.

When you go to bed at night, you don't just get in bed. You take off your clothes, put them away, put on you pajamas, brush your teeth, and say good night to everybody. The computer is a little like that, it needs to 'put on its pajamas' so it will sleep well.

You eject them to make sure nothing is being written at the time. If you pull the cable out while writing you could corrupt the data.

The operating system and the drive itself each have a level of cache to defer writes to the disk and combine them with others to make the process more efficient with less random seeking. USB keys are usually configured with writing cache disabled and are slower, particularly when copying many small files.

When the drive is instructed to turn off, all outstanding data is written to the disk. The heads also get retracted from the surface of the disks and parked on a dedicated ramp. The drive tries to do this in case power is suddenly removed, but won't be able to orderly write out any buffered data.

When a computer is running, it often is reading g data from and writing data to the hard drive. This process isn’t instantaneous, it can take a bit of time to happen. If you unplug a hard drive while data is being written to it in any way, that data will become corrupted and unreadable. If it’s something important like a system file or a file you need, that can be really bad.

Shutting down a computer properly or hitting the “eject flash drive” button always ensures that these readwrite operations finish what they are doing and come to a stoping point where you can unplug the hard drive with no risk of this happening.

Even if you aren’t saving something actively, the operating system does a lot of miscellaneous readwrite operations for all kinds of reasons almost constantly. You never really know when they are happening, so it’s best to be safe with that.

This is also the same reason why unplugging a computer without shutting it down properly is bad. It can corrupt data that is part way through being written to the hard drive.

Your hard drive runs a chaotic, but organized office. It keeps track of a lot of file cabinets, putting your files in them without a system, but always remembering where they are. It's a hectic mess, but your hard drive is a genius and can concentrate pretty well.

Now, it has a bunch of files on it's table, putting some back, pulling some out, all at the same time. It concentrates really hard to remember where everything belongs. Then you come up and bonk it. (Unplugging while it's working) When it wakes up, it might forget where some of the stuff goes and that creates a problem.

Generally, you want to make sure all your components are powered off and cleared of any residual current before touching them, as your filthy lil' hands can make electricity conduct to where it's not supposed to be and bzzt.

Most of modern harddrives have an autopark function. Old drives had to be parked before shutting them off. It's like with a record player: you have to put the pickup arm beside in advance to remove the record iteself. A harddrive has pickup arms, too, and they have to put away before the disk stops. The reason for this: The pickup arms are flying above the disk with the wind the rotating disk produces. When the disk stops, the pickup arm will land on the surface and scatch it and destroy the data written on the disk.

Autopark is a little energy buffer that rips the pickup arms into the innermost part of the disk where they can land without damaging active data.

You don't need to turn off a hard drive to unplug them. The act of unplugging the drive is what turns it off (one of the plugs is for power). Drives can otherwise really only be turned off by the OS's power management plan, but that isn't a pre-requisite for disconnecting it.

That said: any storage media you wish to disconnect while the system is on, you should ensure it not have any pending writes or is not actively being written to. If you disconnect a storage medium while it's in use, you could corrupt its data. Windows has an "eject storage device" feature for external storage devices for exactly this purpose. For internal drives, generally they shouldn't be disconnected while the system is on, even if it's not a problem electrically^([1]); unless the system is specifically designed for it^([2]).

[1] Eg. SATA is hot-pluggable - it can be safely (dis)connected while the system is powered

[2] Eg. servers which can remain operational while dead drives are replaced

Your standard SATA harddrive doesn't need to be turned off. Unplugging is fine if they're going back into the same system. If not? They contain data for another computer that doesn't apply and isn't corroborated by tables.

If you mean why a computer must be turned off, there's a high risk of data loss when a machine is running and there's also a higher chance of frying hardware through touch.