By default, Kubuntu needs you to manually open ("mount") your partitions. In other words, Kubuntu doesn't automount partitions by default. If you like automount (like on MS Windows), you can setup it easily by going to System Settings > Hardware > Removable Storages > Removable Devices > give check the "Enable automatic mounting" > give checks the partitions you want to be automounted under the column "Automount on Login" > OK.

Example: see picture below, all partitions (checked as blue boxes) will be mounted at startup.

How to automount partitions

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