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I have an external HD (WD 2TB My Passport Ultra) that I use with my Mac for Time Machine backup. Everything is working fine. An oddity though: When I eject the drive, the Mac has no complaints, but usually (always?) the HD activity light will continue blinking "for a while" and I can hear the HD heads scurrying back and forth across the platters. Eventually, it quiets down. This all happens when the drive is unmounted, but still connected to the Mac (Disk Utility still shows its existence.) What is happening here? Anything to be concerned about? Do I run the risk of HD corruption if I unplug (and therefore power down) before the drive heads are "quiet"? Something else going on?

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  • The activity LED might not be accurate. It is, in the end, controlled by software inside the external enclosure. It could be programmed to just blink to for 10 seconds after activity.
    – Daniel B
    Commented Feb 25, 2022 at 20:16

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Two operations can account for this activity:

  • The computer emptying modified data from its RAM cache to the disk
  • The disk emptying its built-in RAM cache to the physical rotating disk.

As you see, two caches might be involved in the activity : The computer and the disk itself (which is also a small computer by itself).

Powering down the disk while this is going on is not recommended, because then the integrity of your data depends on the recovery mechanisms that are built into the computer's operating system on the one hand and the disk firmware on the other hand.

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  • Both Windows and Linux write the PC RAM cache to disk before finishing the unmount operation, so macOS would be an oddity here. I can't rule this out though.
    – jaskij
    Commented Feb 26, 2022 at 2:11
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    I would be shocked if the Mac is still writing cached data to the disk - users expect to be able to unplug their external drives safely after the disk icon disappears from the Desktop. And what happens when I have an external SSD? There's no activity lights or sound to tell that R/W are happening. I hope the disk activity is non-destructive after it dismounts...
    – Brie
    Commented Feb 26, 2022 at 23:47
  • All operating systems give priority to flushing external media. Normally modified data is written out within milliseconds, so for you the immediate unplugging is always a possibility. However, if you have just copied a large file to the disk, it may take seconds for disk activity to settle down. At these times, it's not a good idea to unplug the disk. Also, when dismounting, the OS will notify the disk firmware that it's going to be spinned down, which will cause some brief activity as the firmware prepares for its own shutdown.
    – harrymc
    Commented Feb 27, 2022 at 9:38
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Toshiba 5 TB and all my other large external HDDs on RHEL 8 (and earlier versions), following ejection, show prolonged activity for some 20 - 30 seconds. I wait until the control light on the disk indicates the disk is no more active. On an HDD this settling of buffers may take 10x longer than on an SSD and cause vibrations. I am definitely more comfortable if the disk could perform this function.

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