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A few months ago I installed ubuntu alongside windows and everything was working fine. However I only gave the ubuntu partition ~7GB of memory, thinking I wouldn't use it as much. Long story short, turns out I needed a lot more memory for it, so I completely wiped my hard drive, merged all the various partitions I had and started from scratch. I first installed windows. While doing that, from the windows installer I created 3 partitions. 1 - Windows (~300GB), 2 - Various stuff (~50GB), 3 - Ubuntu (~350GB). I installed windows on the first partition with no problem whatsoever. But when I tried to install ubuntu on partition 3, it popped an error saying:

"/dev/sda contains GPT signatures indicating that it has a GT table.      However it does not have a valid fake msdos partitiontable as it should. Perhaps it was corupted possibly by a program that doesn't    understand     GPT partition tables or perhaps you deleted the GPT table and are now using an msdos partition table.Is this a GPT partition table?"

I am pretty new to the world of "partitioning" so I have absolutely no idea what any of that means. So, any kind of help would be immensely appreciated. But please, PLEASE, remember that I'm a newbie to all that stuff, so I probably won't understand very technical solutions. If you kept it simple, you'd have my never ending gratitude. Thank you in advance.

EDIT: I forgot to mention something I think is rather important. When I try to create a partition using the ubuntu installer, it says that I have ~685GB of unallocated space. (I have a 700GB hard drive, so it thinks it's completely empty). I think it doesn't recognize any of my current partitions.

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  • Check if your boot mode is EFI is in BIOS and report back? Also did you install Windows as EFI. ie. How many partitions are there in your Windows installation. Dec 12, 2015 at 8:37
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    First off, thank you very much for the quick reply. Second, I know what none of the things you said are. However I got it to work thanks to another post here in askubuntu.com . (askubuntu.com/questions/249642/…). If anyone else has the same problem, just do what the guy above says and you'll be good to go.
    – KostasKol
    Dec 12, 2015 at 9:06

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First, you're not dealing with memory, but with disk space. The term "memory" refers to random access memory (RAM), which is temporary storage space that's erased whenever you power off the computer. Modern desktop and laptop computers typically have about 4-16 GiB of RAM. Disk space is more permanent, and ranges in size from about 128 GB to 2 TB on modern computers. (Both RAM and disk space can be significantly less than this on older computers or on tablets or other specialty devices; and bigger than these values on high-end computers and servers.)

Your problem was ultimately created because you enabled your Compatibility Support Module (CSM; aka "legacy boot support" or something similar) in your firmware setup utility. You probably did this because you followed somebody's well-meaning but poor advice when you installed Ubuntu the first time, but it didn't bite you until recently -- or if it bit you earlier, you found a workaround. See this page of mine for details. You needn't read that to fix the current problem, but I suggest you do so at some point, or if you run into additional problems trying to solve this one.

In brief, the immediate problem is that Windows has overwritten some of the original installation's GUID Partition Table (GPT) with a simpler and smaller Master Boot Record (MBR) partition table. The result is that you've got leftover GPT data, which is confusing the Ubuntu installer. The easiest solution to this problem is to run my FixParts program on your disk. This tool is part of the gdisk package and should be accessible from a boot of the Ubuntu installer in its "try before installing" mode. FixParts will detect the extraneous GPT data and give you the chance to delete it. After you do so, the Ubuntu installation should proceed normally, with one caveat: If the Ubuntu installer boots in EFI/UEFI mode, it will either complain or try to install an EFI-mode boot loader, which won't work well with your BIOS-mode Windows installation. (This type of mis-match is another possible consequence of enabling the CSM.)

A better solution, though, is to wipe the partition table entirely (use sudo sgdisk -Z /dev/sda from an Ubuntu live CD), disable the CSM in your firmware, and re-install both Windows and Ubuntu. This solution has the advantage that you won't be as likely to run into the conditions that can lead to the problem you've encountered, or other related problems, in the future. Fundamentally, the issue is support for multiple incompatible ways of booting the computer -- BIOS/CSM/legacy vs. EFI/UEFI. Each boot mode has its own unique needs in terms of disk partitioning, and it was a switch from EFI mode to BIOS mode that created the leftover-GPT-data problem you're seeing now. Locking BIOS mode out of the equation by disabling the CSM in your firmware will simplify things and make a whole range of problems go away. You can't do that without either re-installing Windows or jumping through hoops to get Windows to boot in EFI mode, though, which is why this solution will take more effort than the one of removing the stray GPT data and installing Ubuntu in BIOS/CSM/legacy mode.

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