I have being trying to follow the instructions for upgrading to backdrop cms from existing drupal 7 site.  However, I am getting stuck with this error: "Table variable already exists".

I presume this is a table from the drupal dB that has been imported into the backdropdB and it may be left over from my attempts at converting from drupal 7 to 8 which I have now given up on.

I have disabled the three contributed modules I was using in drupal 7: email; IMCE and SecKit.  There appears to be another module Drupal 8 upgrade evaluation which I have also disabled.

What tables need to be imported into backdrop to do the upgrade?


Further information regarding my attempt to 'upgrade' to backdrop:

LAMP versions: Rhel 7.7; apache 2.4.6; Mariadb 5.5; PHP 7.3
1) downloaded and extracted backdrop

2) created new database in mysql with an admin user with correct permissions
    show grants;
    GRANT ALL PRIVILEGES ON `backdrop`.* TO '<user>'@'localhost'

3) created link from backdrop/files to drupal7 files
    lrwxrwxrwx 1 apache apache    30 Sep 30 10:52 files -> /var/lib/drupal7/files/default

4) removed unsupported drupal modules and themes

5) changed settings.php
    $database = '<string>';
    $settings['update_free_access'] = TRUE;

6) imported drupal7 dB tables into backdrop dB
    Current modules/themes list from system table:
    MariaDB [backdrop]> select name from system where status>0;
    | name              |
    | block             |
    | color             |
    | comment           |
    | field             |
    | field_sql_storage |
    | field_ui          |
    | filter            |
    | list              |
    | locale            |
    | menu              |
    | node              |
    | number            |
    | options           |
    | path              |
    | php               |
    | search            |
    | system            |
    | taxonomy          |
    | text              |
    | toolbar           |
    | translation       |
    | update            |
    | user              |
    | standard          |
    | bartik            |
    | seven             |
    | dblog             |

7) setup a dev website on apache

8) attempted url: localhost/dev/update.php
    this switches to using install.php almost immediately which I assume means
    it is unable to access the database or does not recognise that there are updates to be done.
    Forms appear to enter;
    a) language
    b) database, user and password

    a) before adding $database in settings I would receive error re "Table variable already exists." <- so it can access database using those credentials
    b) now I get error from line 3019 in bootstrap.inc about config_... files missing - but this should not happen as MAINTENANCE_MODE should be set to 'update' by update.php (*@#%)(*#@().

OK, I sorted this out.  I had misconfigured apache for the dev environment and it was being redirected back to drupal so it was a drupal error that was appearing.