Say I have several (normal) git-annex replicas. Now, for some reason I want to give one of the machines or hard drives away, so I want to reduce the number of replicas by one, i.e. delete one replica.
The first thing I can do is to ensure by git annex copy . -t other that all the content is present in at least one other replica. Then I can do git annex drop . followed by a git annex sync to remove all the content of the replica I want to delete.
But, what do I have to do to tell the other replicas that this one is gone? Should I just remove the git remote? Or do I have to invoke a special git annex command?
You need to tell one of your other repositories, that this repository is dead. Git annex sync will propagate this information to all other repositories, so all repositories will eventually know that its data is no longer accessible.
After marking it as dead git annex info should no longer list it and sync it.
For some more information, see here:
https://git-annex.branchable.com/tips/what_to_do_when_you_lose_a_repository/