I have a computer with Windows 10. I want to have a hard drive with Ubuntu that Microsoft can't access or read. Can this be done on one computer and two hard drives or do I have to have Windows and Ubuntu hard drives/operating systems on separate computers ?
By default, Windows can not read the
ext4 file system Ubuntu uses at all anyway. To Windows, those partitions have an unknown file system and get displayed as unformatted. Note however that although Windows can not interpret the file system, it can still read it as raw data (e.g. to make a full partition backup image) and overwrite or delete the whole partition, of course.
However, there exist some drivers and applications that allow you to mount
ext4 file systems under Windows as well.
So if all you want is to prevent accidental access by users or spying by Microsoft, applications or Windows malware, you normally don't have to do anything. I have never heard of any Windows malware that include an
ext4 driver just in case someone might have such file systems.
If you're really paranoid, you will have to consider encrypting your Ubuntu partitions, either the full installation (only except some initial bootloader stuff) or only your home directory. That securely prevents any unauthorized user or application from reading your precious data.