>There are two likely sources of an indexing problem
>1) The CDX header is corrupted
>2) The index tag header is corrupted. (The index header contains the what and how to index information), or
>3) The pointers to the records are not correct.
>
>Issuing a REINDEX does not take into account any of the cdx or index header information, the system reindexes on whatever information is currently in the index header and updates the pointers.
>
>The only way to ensure that you have corrected all 3 possible index problems is to issue the DELETE TAG ALL command. This way, the CDX file is actually erased and the file is re-created and index headers re-built when you issue the new "INDEX ON tagexpr TAG tagname" command.

Doesn't cover the fourth possible issue: 4) cdx file is cross-linked, has no header at all or is missing altogether. In any of these cases it's practically impossible to open the table (except that in case of a missing cdx it will forget that it ever had one, and then allow you to open it next time). The recovery utility needs to be able to cope up with whatever happened to the cdx. In FPD I've done it by first blanking the "this table has a cdx" bit in table's header, then physically deleted the cdx, and then created it from scratch. Now if you do a thing like that, the .dbc will surely complain.