If you suddenly disconnect a removable disk from your Mac, you will see this message in the top right corner of your screen: “Disk Not Ejected Properly.” But what does this mean, and why should you eject the drive before unplugging it? Let̵
You must eject before unplugging
If you see the “Disk not ejected properly” message, it means that you disconnected a removable disk before using the macOS software ejection process. The popup tells you to remove the drive “before unplugging or turning it off.”
The eject traces its roots back to removable media such as floppy disks and CD-ROMs that used to be physically ejected from a drive. Early Macs were notable for their use of automatic eject mechanisms (instead of the manual eject button on PCs) that had to be activated within the Macintosh OS software itself.
There are several ways to eject a disc, but the easiest is to select the disc in Finder and choose File> Eject from the menu bar (or hit Command + E on your keyboard). You can also drag the drive to your trash to eject it if you see it on your desktop.
Today, most removable media is not physically ejected, but the command remains a way to warn your Mac that you are about to disconnect a drive. This is why you should do it.
Eject protects your data and your drive
There are three main reasons why it is a good idea to eject a removable disk in macOS before disconnecting it.
When a disk is removable, it means that there is a chance that you will disconnect a disk before any read or write operation is complete, which could potentially corrupt the data. Depending on how heavily your system resources are being used (and how much data you are transferring), these processes can be queued and not complete for some time.
When you eject your drive, you warn macOS that you are about to unplug a drive, and this gives macOS and any applications you use a chance to perform all read and write operations before unplugging it .
The second major reason for ejection is that sometimes your Mac speeds up the apparent writing process to a removable disk by temporarily keeping a copy of the copied data in memory. This is called write caching. Ejecting allows the cached write process to be completed before unplugging so no data is lost. This was a much bigger deal back in the days when USB transfer speeds were slow (and Macs slower, too), but even now you could potentially destroy your data if you unplug the plug too soon after thinking a copy process is complete.
And finally, ejecting ensures that your Mac can safely remove power from the device when all data transfers are complete. For some smaller devices that get their power from the USB or Thunderbolt connection itself, this can be a big problem. For example, suddenly turning off the power to a spinning hard drive can potentially damage the drive. Even flash drives require power to successfully complete writes, and you could pull one out too soon. When ejected, the signal is sent to the disk to shut down gracefully.
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Do I have to eject a disc before restarting my Mac?
The practice of ejecting removable drives has led some to wonder if you should eject a drive before restarting your Mac. The answer is no, you don’t have to eject before turning off or rebooting the device. macOS will automatically complete read and write operations as part of a shutdown or restart.
Windows is slightly different from Mac
If you’re coming to a Mac from a Windows computer, you may be used to being able to quickly remove a drive without ejecting (or “Safely Remove,” as Windows calls it). That’s because Windows keeps write caching disabled by default, so you’re much less likely to lose data as long as no transfer is currently taking place.
On a Mac, there is no option to disable write caching for removable media, so you should always eject it. When it comes to your data, it’s better to be safe than sorry.
RELATED: How to Never “Safely Remove” a USB Drive Again in Windows 10