Your frustration is understandable, but if you end up in the same spot again...
You're not really expected to copy a mnesia dump to another node that way, the expectation is you would add the other node to the dist cluster, then add that dist connected node to the shared mnesia schema (each node can only be in one mnesia schema, but you can have multiple schemas in your dist cluster), and then add table copies for the tables you want, with the storage you want on that mode. You could probably fiddle with the schema directly before starting mnesia to make it work though.
That aside, the data files are accessible via the underlying storage modules, disc_only_copies tables use dets and have .DAT extensions (you'll likely have schema.DAT), and disc_copies use disk log and you'll have .DCD and probably .DCL, LATEST.LOG and PREVIOUS.LOG may exist as well.
Well, it was an old database and we actually just wanted to browse the records to see if there is anything important in them. If I had an old sql backup laying around and want to check whether we will need any of this data in the future the experience is much more straight forward.
The path of mnesia was not so easy and intuitive. :X
You're not really expected to copy a mnesia dump to another node that way, the expectation is you would add the other node to the dist cluster, then add that dist connected node to the shared mnesia schema (each node can only be in one mnesia schema, but you can have multiple schemas in your dist cluster), and then add table copies for the tables you want, with the storage you want on that mode. You could probably fiddle with the schema directly before starting mnesia to make it work though.
That aside, the data files are accessible via the underlying storage modules, disc_only_copies tables use dets and have .DAT extensions (you'll likely have schema.DAT), and disc_copies use disk log and you'll have .DCD and probably .DCL, LATEST.LOG and PREVIOUS.LOG may exist as well.
File type informtion is in the documentation with a pretty reasonable chapter title https://www.erlang.org/doc/apps/mnesia/mnesia_chap7#startup-...