It’s well known that USB flash drives are supposed to be ejected before removing them. Your computer will often display a scary warning if you fail to safely eject the drive. But do you really still need to do this? What happens if you don’t?
First, we need to consider three different protocols your drive may use when connecting to your computer. Two of them don’t need to be ejected at all.
USB Mass Storage: This is the main way a flash drive will connect to a computer. It allows the system to see it as an actual drive — it shows up on the desktop on a Mac, and is assigned a drive letter in Windows. It functions exactly as an internal drive does, so you can use it for storage or even run software off it.
Media Transfer Protocol (MTP): This protocol is built into Windows and requires separate software on a Mac. It’s frequently used by Android devices. With MTP, you can copy files back and forth, but as it is seen by the system as a portable device rather than a drive, it does not need to be ejected.
Picture Transfer Protocol (PTP): This is used by digital cameras to transfer images to a computer. It’s supported by Windows and Mac, and can be used by any application that supports the importing of images. PTP is very similar to MTP, so also does not need to be ejected.
Smartphones, cameras and other media devices that connect via either MTP or PTP can be disconnected simply by pulling out the USB plug without any damage to its storage.
Why Would You Need to Eject a Drive?
If your drive (or device) connects using the USB Mass Storage protocol then it should ideally be ejected. Although, as we’ll see, it differs slightly depending on whether you’re using Mac or Windows. Flash drives today are not just about storing your important files: you can use them to run programs and even entire operating systems.
This applies even if you’re not actually using the device at the time.
The reason is because operating systems use a process called write caching. The operating system doesn’t always write a file to a drive immediately, but instead caches it and waits until it has multiple write operations to complete.
Doing these all at once improves performance, but if the cache is still full when you remove the drive, your data will become corrupted. Clicking the Eject button causes the cache to be emptied and any remaining data to be written to the drive.
That’s the reason why there’s often a delay of several seconds between ejecting the drive and being notified that it’s safe to remove it.
There are also potential issues with the abrupt removal of the power supply from a flash drive. Flash memory is susceptible to this, and it can cause parts of the drive to become corrupt.
Ejecting a Drive on a Mac
All this information applies to flash drives plugged into Mac computers, and to Linux machines as well.
These use write caching to deliver superior performance.
Ejecting a drive on a Mac is a simple process. Just drag the flash drive icon — that will have appeared on the desktop when you first connected it — into the Trash. Or click the Eject icon next to the drive name in a Finder window.
Ejecting a Drive in Windows
The situation with Windows is slightly different.
By default, Windows has write caching disabled. This results in slightly inferior performance when writing to the drive, but also adds a layer of security to the many users who forget to ever eject them.
If you’re not concerned about the risks, you can activate write caching on drive-by-drive basis.
To do this, locate the Device Manager on your PC — Control Panel > Hardware and Sound > Hardware and Printers > Device Manager on Windows 7; or right-click in the bottom left corner of the screen and choose Device Manager on Windows 8.
Right-click on the attached drive, click the Policies tab, change the removal policy to “Better Performance”, and enable write caching setting accordingly.
If you do this, you will always need to ensure you eject the drive via the Safely Remove Hardware option in the system tray.
External Hard Drives
So far we’ve been talking about flash drives. If you’re using an external hard drive then you should always eject it before you disconnect.
Most modern external hard drives will also be powered over the USB connection. Cutting the power while the drive is in use could cause a head crash. This is where the head, which swings back and forth just above the spinning disk to read and write data, comes into direct contact with the disk and causes physical damage to it. This can render large chunks of the disk unusable, or even the entire drive.