First question: What is your physical configuration? To see an "external database" error I would expect you are looking at linked tables. Linked to what? SQL? Excel? DB2? Something that does ODBC?
Second question: How many records do you expect this query to return AND how many fields? When I look up problems with UNION queries, they seem to have some sort of limit based on the combination of the number of tables and the number of fields per table. Sort of like it is the product of the tables times the fields, or it might be the product of the tables times the total field length per record. Something like that. So I am suspicious that you are running into one of "those" errors and the table's host cannot manage whatever it is that you are doing.
Third question: In order to be able to do repetitive SELECT * FROM tableN and then UNION them, that says that EVERY TABLE is structured identically because the fields have to match up exactly to do that. Is that set of identical structures the case?
Because if it is, your REAL solution is to build a SINGLE TABLE that exactly matches the structure of the other tables, then do seven (or however many is needed) INSERT INTO singletable (a,b,c...) SELECT a,b,c,.. FROM tableN queries. Then you need no UNION queries at all. AND if they really ARE identical, you create that query for the first insert, then edit the "FROM" table-name to pick up each source table. One-time operation, should be a piece of cake.