I have a partitioned external HDD (orange LaCie), with one partition for Time Machine. All partitions are Mac OS Extended (Journaled, Encrypted). The HDD is GUID Partition Map.

I'm not (consciously) doing anything with the disk right now, and Time Machine is turned off at the moment.

Why is the light on the HDD blinking (and the HDD active, because I can feel that)? What is it doing?

I am running macOS Mojave 10.14.5.


It's probably the 'Spotlight' indexing that is busy indexing the drive 's content.

  • 3
    Also Activity Monitor.app has a disk tab that tracks this, keeping an eye on that would also tell you what is going on. Jun 29 '19 at 17:59

This could be a few things:

  • In case you chose to encrypt your backups, it may be FileVault2 is currently encrypting the disk. Here you find some instructions how to check that.
  • As In the accepted answer, it could be spotlight indexing the disk.
  • It could be your disk's firmware doing some maintenance task. However, in that case the indicator LED will probably not be flashing. I have a Seagate Archive HDD that uses shingled magnetic recording and practically after any write operation it does a few minutes of reorganising the data. This is done internally by the disk and is intransparent to the user and to macOS.

Another possibility is macOS can be performing a disk check (fsck) in the background. This typically happens if you do not eject the disk properly before unplugging it; then the next time you plug it in, macOS will run fsck in the background on the disk before mounting it. (I'm not sure what the behaviour is for disk formatted with Mac OS Extended (Journaled, Encrypted), but I have experienced this at least with exFAT disks.)

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