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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

2 Answers 2

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

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