W.P. Ginfo on "Problem making dev copy on same server"

This is what I would do [cause it works ;) ]

Step 1: CONTENT

1. FTP: Make a backup of the files of your site, using FTP 'Filezilla' [free] or your own FTP-program.
   (see: http://codex.wordpress.org/Using_FileZilla )
2. Make a backup of the CONTENT of the site (the database):
3. dashboard > tools > export > all content > download exportfile
4. If required: confirm installation of export tool and run the export

STEP 2: Installing the new [dev] site

1. FTP:Copy the OLD installation to the new folder or subdomain.
   !!NOTE!! Do NOT copy the OLD wp-config.php to the new site
2. Browser: open the NEW site > The 5 minutes WP-install starts
3. !! When asked enter the data of the OLD wp-config.php, !!IMPORTANT!! enter a DIFFERENT PREFIX e.g. dev_ (don't forget the _underscore)
   This way both sites sit next to each other in the same database, without problem
4. Import the old database into the dev-site:
5. Dashboard > Tools > Import > wordpress > choose the backupfile from your computer, you just created, and 'upload file and import;
6. If required: confirm installation of importtool and run the import

Done!