I'm trying to install ubuntu from a pen drive. I have windows 7 installed already and want to keep that installation.

I have a 3TB drive that has one 2TB partition on it, so the last 1TB is completely unused, which is where I want to install ubuntu. I started ubuntu in "try ubuntu" mode and then opened gparted, and then deleted the unused partition for the last third of my drive, then tried to install ubuntu.

During the install, it asked me if I wanted to unmount the drives I already have

The installer has detected that the following disks have mounted partitions:

/dev/sda, /dev/sdb

Do you want the installer to try to unmount the partitions on these disks before continuing?  If you leave them mounted, you will not be able to create, delete, or resize partitions on these disks, but you may be able to install to existing partitions there.

No, Yes

I said no because I don't want to lose my windows 7 installation, nor any of that data. I wonder, if I had said yes above, would I have lost all the data on those drives?

Anyways, I hit no and continued.

I chose to install ubuntu alongside windows 7, and hit continue. A few minutes passed when this popup appeared:

Failed to unmount partitions
The installer needs to commit changes to partition tables, but cannot do so because the partitions on the following mount points could not be unmounted:

/media/ubuntu/Three\ Terabyte Drive Terabyte\ DriveDrive

Please close any applications using these mount points. 

Would you like the installer to try to unmount these partitions again?

Go Back, Continue

Why is this not working? What am I supposed to do?

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Update: I went ahead and said yes, it can unmount those partitions. It finished installing Ubuntu, but now when i start my machine it just takes me to the grub rescue prompt. Seems like it broke something.

What can I do now?

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Results of fdisk -l:

Disk /dev/sda: 500.1 GB, 500107862016 bytes 255 heads, 63 sectors/track, 60801 cylinders, total 976773168 sectors Units = sectors of 1 * 512 = 512 bytes Sector size (logical/physical): 512 bytes / 512 bytes I/O size (minimum/optimal): 512 bytes / 512 bytes Disk identifier: 0x00027e14

Device Boot Start End Blocks Id System /dev/sda1 * 2048 206847 102400 7 HPFS/NTFS/exFAT /dev/sda2 206848 976771071 488282112 7 HPFS/NTFS/exFAT

WARNING: GPT (GUID Partition Table) detected on '/dev/sdb'! The util fdisk doesn't support GPT. Use GNU Parted.

Disk /dev/sdb: 3000.6 GB, 3000592982016 bytes 255 heads, 63 sectors/track, 364801 cylinders, total 5860533168 sectors Units = sectors of 1 * 512 = 512 bytes Sector size (logical/physical): 512 bytes / 4096 bytes I/O size (minimum/optimal): 4096 bytes / 4096 bytes Disk identifier: 0x00000000

Device Boot Start End Blocks Id System /dev/sdb1 1 4294967295 2147483647+ ee GPT Partition 1 does not start on physical sector boundary.

Disk /dev/sdc: 16.0 GB, 16008609792 bytes 255 heads, 63 sectors/track, 1946 cylinders, total 31266816 sectors Units = sectors of 1 * 512 = 512 bytes Sector size (logical/physical): 512 bytes / 512 bytes I/O size (minimum/optimal): 512 bytes / 512 bytes Disk identifier: 0x00000000

Device Boot Start End Blocks Id System /dev/sdc1 * 32 31266815 15633392 c W95 FAT32 (LBA)

Disk /dev/sdd: 999.5 GB, 999501594624 bytes 255 heads, 63 sectors/track, 121515 cylinders, total 1952151552 sectors Units = sectors of 1 * 512 = 512 bytes Sector size (logical/physical): 512 bytes / 512 bytes I/O size (minimum/optimal): 512 bytes / 512 bytes Disk identifier: 0x0002ae3f

Device Boot Start End Blocks Id System /dev/sdd1 2048 1952151551 976074752 7 HPFS/NTFS/exFAT