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Often this will be because the drive is being indexed; right click the drive>properties, and uncheck index. Or the AV is monitoring the drive - turn off AV to test this.

There are several reasons to USB being undismountable:

1. It's in use. Please note that if you're looking at the disk in Explorer, then it's in use!


2. Windows is finishing copying a big file to or from the disk (rarely the reason)


3. A bug in Windows causes conime.exe to get stuck on the disk (it may safely be killed)


4. If the USB drive is formatted as NTFS, the journal of all file updates may still be open


5. If disk indexing is on for the drive, Windows may be furiously indexing all files on it

My advice is to:

1. Make sure the USB drive is not defined as indexed.


2. Wait, and then try again. It might work a few seconds later on.


3. Buy Zentimo ($29.90) that does a good job of trying to remove the disk. Even if it fails, it still tells you which programs are using what files (I paid).

This is by no means a good answer, but you can always shut down the computer and remove the USB device.

I found the free USB Disk Ejector to be very useful:

A program that allows you to quickly remove drives in Windows. It can eject USB disks, Firewire disks and memory cards. It is a quick, flexible, portable alternative to using Windows’ “Safely Remove Hardware” dialog.

If it's none of the pedestrian/common options, it could be you have the drive listed as 'shared' on a network and have accessed it via another computer. In my case, even though the remote computer was off, it still had a lock on the directories. Unshare and it should work if that's the case.

• Right click on the properties of the USB drive in Windows Explorer.


• In the popup window click on "Hardware".


• Identify the drive and left click on it once to highlight it, then click the properties box.


• Click on "Change settings".


• Click on the Policies tab and select "Optimize for quick removal" and click OK.

If you're sure that none of your own processes are still in use on this USB just pull it out.

[Edit: Not found official Microsoft documentation of this method, but Cnet has an article about it: Quickly remove USB devices without using Safe Removal .]

Quick Way to Find Process/File Open that is Preventing Unmount:

I just figured this out. With Process Explorer (free download) this is really easy.

1. Download, unzip and run procexp.exe
   


2. Choose from the "Find" menu, "Find Handle or DLL..." or hit Cntl-F


3. Enter only the drive letter (followed by ":") in the search text box

This will show you all the open files on your removable volume, the processes that own the file handle and the PIDs of the processes. Double click to highlight the process in the main window (top) and file (bottom). From there you can right-click the process to kill it or right-click the file to close the file handle.

I think this is because Windows itself is still finalising the writes to the hard drive. I get the same message if I try to disconnect straight after copying some files. If I wait until the drive has stopped spinning then it disconnects without any problem.

I haven't been able to find any information on why this is, so if someone could enlighten us I'd appreciate it.

Take a look for programs installed recently, which can prevent safe HDD disconnection.

In my case it was Magical Defrag 2: any newly connected disk with new letter assigned was automatically marked, so now after unmark I can safely remove the USB drive.

Simple fix (Windows 7 Professional 64-bit):

1. Change the drive letter, and if you want to keep the same letter, change it back.




2. The drive can now be unmounted.

When you change the drive letter, it disconnects the drive from all processes that might be using it, as the warning message states. Once that happens, you can then unmount it the usual way.

If you don't know how to change drive letter, follow these instructions.

1. Control Panel --> Administrative Tools --> Computer Management -->
   Storage --> Disk Management (wait ~10 seconds for information to appear.)




2. Select the problem drive under volume heading.




3. Scroll to that drive in the lower part of window.




4. Right-click on the drive, and select Change Drive Letter and Paths.




5. Change the drive letter to one not reserved for another drive.




6. Acknowledge the warning message.

If you wish the drive to retain the original letter, simply switch it back. The drive can now be unmounted.

The 2 most reliable ways are ones that nobody has suggested yet because they are sometimes the most inconvenient:

1. Log off. Log back on. Try again to safely eject the USB device (via the Windows tool). If even that didn't work, then:


2. Shut down your computer entirely. Then simply unplug the USB device without asking permission.

This is due to the fact that there is at least a process/program that is still accessing your USB drive. It may not be enough if you just close all the programs on dekstop, if you are copying folders from/to the USB hard disk, you will get this message.

Try and find what process is holding the resource and either complete or close the process and you should be able to remove the USB hard disk without any problems.

Do let us know how it goes.

I've noticed this a lot. What we do is to share and then unshare the drive. It's had a 99% success rate. I have no clue why it works, but time to time it says another user is accessing the drive... even though I just shared it only to unshare it. Try that. You'd be surprised at the success rate.

For me changing the drive letter did it. I could not kill the accessing process cause it was PID 4 (system itself), which opened H:$EXTEND..... (did see it in Process Explorer).

After changing the drive letter i could savely remove it.....
WOW, Windows is a never ending adventure game..... :))))

this method worked to me
1) : Run -> "diskmgmt.msc" -> Find your drive in the physical disk list (bottom pane) -> right-click the drive (the left-most part), not the partition -> Click "Offline"

2) : Run -> "cmd.exe" -> type "diskpart" -> type "list disk", find your disk # -> type "select disk x", where x is your disk's # from the last step -> type "offline disk". Now you can exit the command prompt or just type "exit" at diskpart, then close the prompt."

I use "remove all dismount" on the diskpart command to kick the disk out. can't make it simpler now as Devcon command couldn't kill the disk out soon either.

now use the safely remove hardware and eject media tool

Thanks to

Reizaal Abdul Aziz

https://answers.microsoft.com/en-us/profile/bf1a3358-a39a-4c32-bc7e-692576439363

Thank you all for this windows problem that many have problems with, this is my answer to it.
Easy : not click the safe remove icon on taskbar but goto the USB stick in explorer and Eject there, Solved.

Try hotswap free utility, you can download here http://mt-naka.com/hotswap/index_enu.htm.

• Ensure to activate Safely removable devices by right click the icon hotswap on systray - device type - Safely removable devices.


• Then you can remove USB drive by left click icon hotswap on systray and select your removable drive.