After upgrading a site from D5 to D6, I installed the imagecache module to better handle my thumbnails, but it wasn't working. I had created a preset, created an imagefield, selected the preset in the display fields -- but Drupal created neither the image files nor the directories. Both /files and /temp were 777.
So in /admin/settings/file-system I created a new /files_temp directory. Turns out the .htaccess file was different. I copied the new /files_temp/.htaccess over the old /files/.htaccess, switched the /admin/settings/file-system directory back to the old /files, and it worked.
To save all that back-and-forth, just replace the contents of the old .htaccess file:
Options None
<IfModule mod_rewrite.c>
RewriteEngine off
</IfModule>
with the contents of the new one:
Options None
Options +FollowSymLinks
