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I installed Ubuntu OS. Then tried to install Windows 7 as dual boot. But i get error message that unknown file system. Not NTFS. Now there is no OS in System. How to change the hard drive file system to NTFS to install Windows 7 and then Ubuntu?

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You can do it from any installation media (ubuntu/windows 7).

Take a backup important data using any live OS (like ubuntu). Following procedure will delete the data in your system. You can not change file system type without deleting data.

Since you want to install windows 7 first, do the following steps..

  1. Boot using windows 7 dvd/usb.
  2. Choose "Custom Installation" (not upgrade)
  3. It will show a list of partitions available in your system. Filesystem other than FAT/NTFS will be shown as unknown file system
  4. Below the list, there will be an option written as Advanced, click that it will open hidden options.
  5. From here, you can delete the partitions and recreate (All data will be lost but will be fresh system), or do a format. The formatted disk will be ntfs. If formatting repeat the steps for other drives (which are not ntfs).
  6. Now choose the first partition (or partition of your choice) where you want to install windows.
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