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I want to dual boot my laptop(lenovo G580). It currently has windows 8.1(unactivated). Now I also want to install ubuntu 14.04 64-bit, so basically I was trying to dual boot using bootable usb drive. When I first try booting ubuntu, in the process, it says no previous OS installed i.e., its not recognizing windows 8.1, so I search google and somewhere found that I need to disable UEFI mode. After disabling UEFI mode I again tried to install ubuntu, but this time the usb stick is not getting recognized(or OS inside is not getting recognized, bcoz the boot manager recognizes HDD and tries to boot from it) and this below statement on a black screen comes out and then nothing happened.

"An operating system wasn't found. Try disconnecting any drives that don't contain an operating system.

Press any key to restart. "

Please some one help I want to install ubuntu over windows. Also, please do tell me if any other info is required!

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The advice you received to disable UEFI-mode booting was probably bad. Pre-installed Windows 8 and later are almost always installed in EFI/UEFI mode, and setting up a BIOS/EFI-mode dual-boot is almost always much more hassle than setting up an EFI/EFI-mode dual-boot. In other words, you're creating problems for yourself by doing what you did.

An exception to the preceding is if you installed (or re-installed) Windows 8.1 yourself. In this case, if you did the installation in BIOS/CSM/legacy mode, it could be you've got leftover GPT data on an MBR disk. This Web page of mine covers this topic.

Your initial problem ("it says no previous OS installed") is probably just a matter that requires use of the "Something Else" installation option, as described in this askubuntu.com question and answers. If you get into the partitioner and it shows the disk as being unpartitioned, though, then you've got some other problem -- probably either stray RAID data or partition table trouble. See my Web page, referenced earlier, for information on such problems. If you suspect partition table trouble and you aren't sure how to proceed, you'll need to post more detailed information, such as the output of:

sudo gparted /dev/sda print
sudo gdisk -l /dev/sda
sudo fdisk -l /dev/sda

These commands all assume you've got one disk (/dev/sda). If you've got multiple disks, you must repeat each command for all your disks.

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