How do I dual boot Kubuntu with an existing non-OEM install of Windows 10?

Question

A bit of history on this computer:

I built this computer in 2015 back before Windows 10 was released. The BIOS accepts both UEFI and non-UEFI devices as bootable media and I turned off Fast Boot quite soon as I found it annoying. I had the option of getting either a Windows 7 or Windows 8 installation disk and opted for the Win 7 one because:

1. I already had a laptop that came pre-installed with Win 8 and didn't like its start menu garbage and
2. I had heard rumors that Win 8 was less secure than Win 7.

A couple of years later when Windows began to offer a free upgrade to Win 10, I decided to accept the offer and upgraded Windows.

Back to the present:

I have recently decided that I want to install Kubuntu 18.04 LTS in a dual boot configuration with my current install of Win 10. I have managed to shrink the C:\ partition of my drive to half its original size but I can't manually add the partitions necessary to follow other installation procedures that I have found as the unallocated region becomes "unusable" after I add a single partition during installation.

Requested Information:

Output of sudo parted --list:

Model: ATA WDC WD10EZEX-00B (scsi)
Disk /dev/sda: 1000GB
Sector size (logical/physical): 512B/4096B
Partition Table: msdos
Disk Flags:
Number  Start   End     Size   Type     File system  Flags
 1      1049kB  106MB   105MB  primary  ntfs         boot
 2      106MB   500GB   500GB  primary  ntfs
 3      1000GB  1000GB  472MB  primary  ntfs         diag
Model: TOSHIBA External USB 3.0 (scsi)
Disk /dev/sdb: 3001GB
Sector size (logical/physical): 4096B/4096B
Partition Table: msdos
Disk Flags:
Number  Start   End     Size    Type     File system  Flags
 1      1049kB  3001GB  3001GB  primary  ntfs         boot
Model: SanDisk Cruzer Blade (scsi)
Disk /dev/sdc: 8004MB
Sector size (logical/physical): 512B/512B
Partition Table: msdos
Disk Flags:
Number  Start   End     Size    Type     File system  Flags
 1      1049kB  8004MB  8003MB  primary  fat32        boot, lba

Output of sudo mount:

sysfs on /sys type sysfs (rw,nosuid,nodev,noexec,relatime)
proc on /proc type proc (rw,nosuid,nodev,noexec,relatime)
udev on /dev type devtmpfs (rw,nosuid,relatime,size=4034560k,nr_inodes=1008640,mode=755)
devpts on /dev/pts type devpts (rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode=000)
tmpfs on /run type tmpfs (rw,nosuid,noexec,relatime,size=811152k,mode=755)
/dev/sdc1 on /cdrom type vfat (ro,noatime,fmask=0022,dmask=0022,codepage=437,iocharset=iso8859-1,shortname=mixed,errors=remount-ro)
/dev/loop0 on /rofs type squashfs (ro,noatime)
/cow on / type overlay (rw,relatime,lowerdir=//filesystem.squashfs,upperdir=/cow/upper,workdir=/cow/work)
securityfs on /sys/kernel/security type securityfs (rw,nosuid,nodev,noexec,relatime)
tmpfs on /dev/shm type tmpfs (rw,nosuid,nodev)
tmpfs on /run/lock type tmpfs (rw,nosuid,nodev,noexec,relatime,size=5120k)
tmpfs on /sys/fs/cgroup type tmpfs (ro,nosuid,nodev,noexec,mode=755)
cgroup on /sys/fs/cgroup/unified type cgroup2 (rw,nosuid,nodev,noexec,relatime,nsdelegate)
cgroup on /sys/fs/cgroup/systemd type cgroup (rw,nosuid,nodev,noexec,relatime,xattr,name=systemd)
pstore on /sys/fs/pstore type pstore (rw,nosuid,nodev,noexec,relatime)
efivarfs on /sys/firmware/efi/efivars type efivarfs (rw,nosuid,nodev,noexec,relatime)
cgroup on /sys/fs/cgroup/cpu,cpuacct type cgroup (rw,nosuid,nodev,noexec,relatime,cpu,cpuacct)
cgroup on /sys/fs/cgroup/blkio type cgroup (rw,nosuid,nodev,noexec,relatime,blkio)
cgroup on /sys/fs/cgroup/perf_event type cgroup (rw,nosuid,nodev,noexec,relatime,perf_event)
cgroup on /sys/fs/cgroup/rdma type cgroup (rw,nosuid,nodev,noexec,relatime,rdma)
cgroup on /sys/fs/cgroup/cpuset type cgroup (rw,nosuid,nodev,noexec,relatime,cpuset)
cgroup on /sys/fs/cgroup/memory type cgroup (rw,nosuid,nodev,noexec,relatime,memory)
cgroup on /sys/fs/cgroup/freezer type cgroup (rw,nosuid,nodev,noexec,relatime,freezer)
cgroup on /sys/fs/cgroup/net_cls,net_prio type cgroup (rw,nosuid,nodev,noexec,relatime,net_cls,net_prio)
cgroup on /sys/fs/cgroup/devices type cgroup (rw,nosuid,nodev,noexec,relatime,devices)
cgroup on /sys/fs/cgroup/hugetlb type cgroup (rw,nosuid,nodev,noexec,relatime,hugetlb)
cgroup on /sys/fs/cgroup/pids type cgroup (rw,nosuid,nodev,noexec,relatime,pids)
systemd-1 on /proc/sys/fs/binfmt_misc type autofs (rw,relatime,fd=26,pgrp=1,timeout=0,minproto=5,maxproto=5,direct,pipe_ino=15448)
mqueue on /dev/mqueue type mqueue (rw,relatime)
hugetlbfs on /dev/hugepages type hugetlbfs (rw,relatime,pagesize=2M)
debugfs on /sys/kernel/debug type debugfs (rw,relatime)
tracefs on /sys/kernel/debug/tracing type tracefs (rw,relatime)
configfs on /sys/kernel/config type configfs (rw,relatime)
fusectl on /sys/fs/fuse/connections type fusectl (rw,relatime)
tmpfs on /tmp type tmpfs (rw,nosuid,nodev,relatime)
tmpfs on /run/user/999 type tmpfs (rw,nosuid,nodev,relatime,size=811148k,mode=700,uid=999,gid=999)

Answer 1

Please back up before doing the partitioning using CloneZilla Live

You're in luck:

You have 3 primary partitions on your 1TB drive with an MS-DOS partition table (4 partitions being the maximum):

Partition Table: msdos

Number  Start   End     Size   Type     File system  Flags
 1      1049kB  106MB   105MB  primary  ntfs         boot
 2      106MB   500GB   500GB  primary  ntfs
 3      1000GB  1000GB  472MB  primary  ntfs         diag

So what you have to to is to:

• create one extended partition on the free space between Partition 2 and 3 (taking all that space, nearly 400GB)
• create logical EXT4 partitions inside the extended partition:
  • as many and of the size you like
  • grub and Ubuntu don't care whether they are on a primary or logical partition, whereas Windows 7 did and as you've upgraded, that's how Windows 10 is still configured.