It is not possible to recover user databases from c2. While usually I recover them manually, in this case there was an unrecoverable disk failure.

This was a conscious decision- these are replica databases, and the only reason they are allowed to be written is for temporary tables in order to join with production data. There is no backups and no replication for user databases on c1, c2 and c3. See, for example my answer on T56401#1735958 a few months ago:

Most user databases there are used for scratch data, and that is the intended usage. There has been never a guarantee of high availability of those databases, while disk space is provided if offline copies are required (each user's responsability). This is the same model than code: you should have copies on a WMF or external repository.

Because there are 3 physical servers, which may need maintenance at any time, you must save elsewhere all your data and be prepared for it to be reimported or regenerated. We provide space for that. If you need help setting up database backups, I can help.

If you want to host a database with (some) redundancy for user tables, toolsdb is the right place:

User-created databases can be created on the database hosting the replica servers or on a database local to the 'tools' project: tools-db. The latter tends to be a bit faster since that server has less heavy activity, and tools-db is the recommended location for user-created databases when no interaction with the production replicas is needed.

If the expectancy was different than that, I apologize and will document it better.