Too many Ubuntus

When you delete /dev/sdb5, the other logical partitions will be renumbered. Because of the renumbering that will take place, you can't have any partition with a number > 5 loaded, so you can't delete it from your Ubuntu on /dev/sdb7 -- so you'll need to do it from the Ubuntu CD, Windows, etc.

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Also, you'll want to make sure your /etc/fstab is mounting by UUID -- because /dev/sdb7 will be something else (/dev/sdb5 or /dev/sdb6) after you delete the /dev/sdb5 partition.

From a terminal,
>> sudo gedit /etc/fstab
In your /etc/fstab, if a line begins with "/dev/sdb7", then you need to replace it with "UUID=xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx". Leave the rest of the line alone. If no lines have "/dev/sdb7", then you're already good to go.

If you need to determine the UUID of /dev/sdb7, open a new terminal:
>> blkid | grep sdb7

You should get an output like:
/dev/sdb7: UUID="xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx" TYPE="ext4"
Copy the UUID hexadecimal code to /etc/fstab.