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Cannot unmount disk to format it

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I failed to install Lion on my pc. Now I am trying to install win8 on it but it wont let me cos of the GPT so I tried to erase my disk to Ms-dos FATso i can install win on it but it stucks at Unmounting disk. I am booting from my DVD

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if you can get into command prompt , run as an administrator, run following commands

diskpart     enter

list disk     enter

you should have lists of hdds on pc as



disk1 ..............check one with gpt partition(for example  disk1)


 select disk1    enter,    once gpt disk selected

clean enter               and thats all.

if you have another windows pc , you can connect hdd to it and do as above.


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Run your Lion DVD, open Terminal and enter this.


sudo diskutil umount -f /Volumes/YourPartitionName


Close Terminal, open Disk Utility. (Still in Lion Installer)

Format it FAT, then run Windows DVD and format to NTFS/FAT.


Make sure the drive is MBR for Windows DVD as well.


* If that Terminal command doesn't work then try: 

sudo diskutil umount force /Volumes/YourPartitionName

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