There is a lot of information about how to safely unplug USB hard drives. However, the articles tend to stop at the part where the operating system says it is safe to unplug, or where you're sure nothing is writing to the drive, etc.
Relevant set up:
- Windows 10 PC or Windows 10 notebook
- External USB hard drive (not a USB stick)
- Click "Eject External USB 3.0"
- Wait for "Safe To Remove Hardware" notice
After the operating system declares that it is safe to unplug a USB hard drive, the LED on the drive blinks about 10 times before the drive shuts down.
I've used a number of external 1TB to 4TB HDDs by Toshiba and Seagate, and they all consistently blink their LEDs about 10 times when ejected. The blinking happens after the OS says it is "Safe to Remove Hardware". Although the OS is limited to seeing the completed buffer-flush whereas the drive's firmware can detect when the writes have completed, the 10 blinks happen even when the drive is just plugged in and immediately ejected with no writes at all. Likewise, it's still the same 10 blinks when the drive is written to extensively, then ejected.
Some theories:
The 10 blinks are just a courtesy to make it easier to identify which drive was ejected when multiple drives are plugged into a single computer.
There is a fixed latency between the OS flushing its write buffers and the writes getting physically encoded. This would explain why any non-zero amount of writes produces a fixed delay on eject. However, the 10 blinks still take place if there was a long delay between writing and ejecting, or if there wasn't any (intentional) writing at all.
The 10 blinks represent a safety margin for the drive to park its heads. That sounds like a rather long time just to park heads, though, with each blink lasting about a second.
So, is it safe to unplug the hard drive while its LED is still blinking?
I've tried finding an authoritative answer, even checking on Seagate's and Toshiba's websites, looking at the datasheet in the case of Toshiba. I'm looking for answers that reference credible sources regarding what the drive does during the shutdown blinking sequence, to understand whether unplugging during that sequence is safe.
Here are some other areas I checked:
Searching Google: is it safe to unplug flashing usb
I got a whole bunch of tips about how to safely unplug USB drives, but nothing to do with the USB.
Searching Google: is it safe to unplug blinking +LED usb
Some other posts come up, mainly to do with troubleshooting, but nothing to do with the routine 10 blinks on eject.
Searching Super User: [usb] [hard-drive] [windows-10] eject
This produces 7 results (or 8 results if I replace [hard-drive] with [external-hard-drive]), mainly about problems. Nothing about the routine 10 blinks on eject.