External HDD is always in use when trying to safely remove

As explained by Rook in this answer: https://superuser.com/a/674255/142560

To safely remove the drive:

1. Open Command Prompt (cmd.exe).
2. Type diskpart.
3. Type list disk.
4. Find your disk #, and type select disk [number here].
5. Type offline disk.

You should be able to remove it now.

Next time you plug it in, it won't be automatically mounted. So either use the command prompt again to make it online, or:

1. Run diskmgmt.msc.
2. Find the disk, right click, and choose "Online".

Got the same problem with a different external disk used for backup of the system disk. I identified the TxF file locks using LockHunter, which wasn't able to unlock them.

I hear Transactional NTFS is used by autoupdate, but have no clue why the system would want to place this on an external disk and then be unable to stop it upon safe removal request. Fsutil resource info doesn't show any activity.

Try in the cmd console:

fsutil resource stop E:

or, if it doesn't help,

fsutil resource setautoreset true

and reboot. You can also try stopping a TxF-related service in Computer Management / Services

Windows 7 is trying to Backup to your external Hard drive, You can disable backups to external disk drive. Sample solution is here.