[xubuntu] how to "mount" HDD

Secure boot has nothing to do with it. But Windows 8/Windows 10 use something called "fast startup" which is a form of hibeation - and that means the file system as last used by Windows could be in an unstable state if you tried to access it without shutting down properly. Sounds like you can't boot up Windows, obviously, to try to disable fast startup.

You might be able to mount the partitions read-only in a terminal window. Try this:

Open a terminal windows (Ctrl Alt t)

When the window opens. type this command:

sudo mount -t ntfs-3g -r /dev/sda4 /mnt

ls /mnt