.htaccess redirect subfolder to HTTPS

This work for me, this allow you to redirect to https a specific folder, just add an htaccess file inside of the folder with the following content:

RewriteCond %{HTTPS} off
RewriteRule .* https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301]

I would put this into the domain's root directory:

RewriteEngine On
RewriteCond %{HTTPS} !=on
RewriteRule ^(subdirectory/.*)$ https://www.mydomain.com/$1 [R=301,L]

Ahh, its a combo of the two. MonoMano - you've omitted the subdirectory in the first part of the RewriteRule, therefore directing ALL traffic to the HTTPS subdomain address. I found that checking for 'off' was more reliable than checking for !=on, dont ask me why!

You'd want to add the subdirectory back in as per Floern's response, see below:

RewriteEngine On
RewriteCond %{HTTPS} off
RewriteRule ^(subdirectory/.*)$ https://www.mydomain.com/subdirectory/$1 [R=301,L]

Add the lines given below to .htaccess file of that subdirectory:

RewriteCond %{HTTPS} off
RewriteRule ^(.*)$ https://www.domain.com/subdirectory/$1 [L,R=301]