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Running Windows 10 with two 16GB pen drives (mapped to F: and P:) and a larger 4TB WD external disk (mapped to W:). All drives work correctly, appear in Windows Explorer, Disk Management, you can navigate to them in the command prompt, etc.

But the external hard drive is not appearing in Optimise Drives (right-click any drive > Properties > Tools > Optimise and defragment drive). The two pen drives do appear.

Now open a command prompt and type defrag W: /a. The defrag utility instantly returns with no analysis or error. Type defrag P: /a and some analysis stats are displayed. Type defrag M: /a and the following error is displayed:

The given volume path is invalid. (0x89000001)

The external hard drive W: appears to be recognised, but ignored, by the defragmentation utilities built into Windows.

Why? And how do you defragment a drive that Windows won't show?

Update

From the command-prompt I ran "chkdsk W: /f" and after completion (took about 3 minutes), the extrernal drive appeared in Optimise Drives. Still curious to know if there is more going on here.

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There are a couple listed reasons a drive might not show up in the Optimize menu (dfrgui.exe):

You can't defragment specific file system volumes or drives, including:

  • Volumes locked by the file system.

  • Volumes the file system marked as dirty, indicating possible corruption. You must run chkdsk before you can defragment this volume or drive. You can determine if a volume is dirty by using the command fsutil dirty query C: .

  • Network drives.

  • CD-ROMs.

  • File system volumes that aren't NTFS, ReFS, Fat or Fat32.

https://learn.microsoft.com/en-us/windows-server/administration/windows-commands/defrag

In your case, it's most likely the drive was in dirty state (which usually shows a different error in defrag), or some other problem with the filesystem fixed by chkdsk. Some external or USB HDDs have a kind of "sleep" mode for power saving you can look into too, though it's less common.

If it happens again, you can try to unmount/remount the drive, or reboot windows with the drive connected

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