It is likely a permissions issue. Kubuntu and Ubuntu use AppArmor for
security and recent versions of both have tightened control. It depends on
the file system format and how it handles permissions about how many hoops
you have to jump through.

If it is formatted FAT32 all you have to do is open it and it will
automount. Once it is mounted you can use it in any application.

If it is a Linux file system things are tighter. You will have to provide a
password if the file system was created outside of your user space. Or you
can change the permissions so that it has you as user and users as the group
with full write access.

The other thing that sometimes confuses people is if you open a file manager
as root home will be root's home and not your own. You will have to navigate
to your own home. Also any changes that you make will only be available to
root unless you change it to your user name and change the group
accordingly.

I have KDE set up to mount my partitions when I login. It prompts me for
passwords for my ext4 data partitions. It is a bit of nuisance up front, but
it allows me to use all of my partitions without any delays or
inconvenience.

In GNOME I just wait until I need them and then click on the partition and
it will ask for passwords as needed.

If you don't mount them then they will be unavailable to you in applications
until you mount them.