I had a problem which I fixed and am posting it here just incase anyone else has the same problem.
My situation: I was developing a site on my server with a directory path of "/sitename/". I successfully transferred it over to a new server with the directory path of "/" but I got the 500 internal error on every page EXCEPT the homepage.
The .htaccess file read:
# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /sitename/
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /sitename/index.php [L]
</IfModule>
# END WordPress
The fix:
RewriteBase /
and
RewriteRule . /index.php [L]
(just took out the "sitename" so it pointed to the root directory). Simple problem and simple fix, just thought it may help if you forget like I did.