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Digikam now allows for multiple albums, including albums on removable media. However, the tagging system has not kept up with this. If you apply tags only to files in a removable device, and remove that device, you still have all of the tags even though there are no pictures associated with it.

I see two ways to fix this.

One is that digikam keeps track of which albums use a given tag. If all of the albums containing that tag are absent, then the tag is hidden. The disadvantage of this is that you might not want to lose that tag. A solution would be to have an option on right-clicking a tag to keep it even if there are no pictures in it.

The other option is that within the list of tags there is a group of tags for each album and another group of permanent tags. Tags that are in a given album disappear when the album is not available. Tags that are in the permanent category remain even if there are no albums. The disadvantage of this is that you cannot have tags that span multiple removable albums but don't stay when those albums are removed.