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Hi I wanted to reformat my HP 500GB hard disk to MS DOS (FAT) by using Disk Utility but Disk Utility kept saying that the hard disk cannot be unmounted. I tried googling it but nothing seems to answer my question. I'm not trying to reformat the hard disk in my Mac but my external hard disk!

I usually format my hard disks to clear the documents I no longer need. And I'll change it to ExFat but currently, whenever I plug in my hard disk on Windows, the whole system just hangs there.

Can someone help??

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you should try it using linux. or boot from ultimate boot disk to format it. – Hemc Mar 11 '13 at 6:25
@Hemc Installing linux just to format a disk on Mac seems an overkill. – Matteo Mar 11 '13 at 6:37
@Faith Most probably some process has an open file on your hard disk. You could try to reboot. – Matteo Mar 11 '13 at 6:39

You can't have any files that are located in the external hard drive opened when trying to format it, nor any program installed on it running. Try unplugging and then plugging it, then immediately formatting it.

Assuming you're on OSX, if you're still unable to unmount it, you can try forcing an unmount by issuing the follow command:

sudo umount -f /Volumes/External_HD_Name

Alternatively, try forcing an unmount with disk utility:

sudo diskutil umount force /Volumes/External_HD_Name

Replacing External_HD_Name with the your disk's name.

Alternativly you can prevent a device from being automatically mounted following this tutorial. You can also burn GParted to a CD, boot it and try to format your external HD from it.

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Yes, ensure no "disk prep" apps are open for the external drive. Many come preloaded with Win and OS utities that must be closed. That fixed it for me. 😜

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I must be missing something ... Why do you suspect that Windows OS utilities would be open on the OP's Mac? – G-Man Oct 21 '14 at 21:46
What is a "disk prep app"? Do you have an example for one? – user 99572 is fine Oct 21 '14 at 23:17

I used both commands. Upon using the second command, I got an error stating that the disk couldn't be unmounted. Nothing was using the HDD at the time.

caelenwilson$ sudo umount -f /Volumes/Caelen_HDD

WARNING: Improper use of the sudo command could lead to data loss
or the deletion of important system files. Please double-check your
typing when using sudo. Type "man sudo" for more information.

To proceed, enter your password, or type Ctrl-C to abort.

umount: /Volumes/Caelen_HDD: not currently mounted
caelenwilson$ sudo diskutil umount force /Volumes/Caelen_HDD
Umount failed for /Volumes/Caelen_HDD


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This does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post - you can always comment on your own posts, and once you have sufficient reputation you will be able to comment on any post. - From Review – nc4pk Dec 30 '15 at 17:33
@nKn Thanks for attempting to improve the post. However, editing causes all not-an-answer flags on the post to be dismissed and takes the post out of the Low Quality queue. The answer is still very much not an answer (and I can't re-flag as NAA), so in such cases it's better to just Recommend Deletion, unless you can make it an answer. – Ben N Dec 30 '15 at 20:27
Oops... didn't know that, sorry. I'll take this into consideration for the next time! – nKn Dec 30 '15 at 20:29

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