If during ubuntu installation you don’t see your existing partitions but all free space then there might be two possibility.
1. You have both GPT and MBR partition tables present on your hard disk.. It is generally caused when you install different windows in the past which used a different partition table scheme (MBR or GPT). So you end up with both Schemes. So we must make all partition scheme the same.
Check if you have multiple partition schemes with this command
sudo gdisk -l /dev/sda
In the screenshot below, It shows that MBR and GPT both are present.
In order to fix it.. we use this tool, fixparts, enter the following command
fixparts /dev/sda
Its telling me that GPT signature is left on my current partion table and is of no use and asking me to remove it.
Enter ‘Y‘ to remvoe it.
Note: you might get a different message depending upon you situation.
Now, we get to enter the MBR command to write the partition table to disk, which is ‘w’
So go ahead and enter w. (you can see all MBR commands by entering ‘?‘ )
Done!
You can verify if all partitions are in the same scheme by again entering the first command
sudo gdisk -l /dev/sda
2. The other possibility is that you have dynamic partitions, this is common for windows 10 installation. So in this case you will need to format the whole hard drive and make it to simple partition.
and then try the ubuntu installation.