.htaccess file “not writable”

When trying to change the permalinks for my blog, I receive a message that says: “If your .htaccess file were writable, we could do this automatically, but it isn’t so these are the mod_rewrite rules you should have in your .htaccess file. Click in the field and press CTRL + a to select all.”

I am fairly certain that my .htaccess file IS in fact writable. Here is some information that may be relevant:

• The WordPress core files have been moved to a different directory than they were originally installed in, and the WordPress Address and Blog Address have been updated to the correct location in the General settings.
• The index.php and .htaccess files are located at [webroot]/Blog, and the core WordPress files are located at [webroot]/wp
• The .htaccess file permission is set to 644. I have tried it as high as 777 (don’t worry, I changed it back), but that did not allow it to be modified by WordPress.
• The only major difference between this particular installation of WordPress and another one (in a different location) on my server is that the WordPress core files are in a different location from the main index.php and .htaccess files.

I did find another post in this forum from a few months back where the same problem was described, but I did not see a solution. That post is here. One difference between this post and my setup: I currently do not have any plugins installed (other than the 2 delivered ones) or activated.

I am still able to manually edit my .htaccess file, so I am not completely unable to change my permalinks, but I would like figure out why they are not writable by WordPress, when it appears (to me) that they should be writable.