Upload error - The file could not be written to disk

[eluser]omar_yoi[/eluser]
I am using this code to explain the path to the images

Code:

$this->gallery_path = realpath(APPPATH . '../img');

I have tried to removed one dot (.) or both, however the following error message appears:
The upload destination folder does not appear to be writable.

otherwise if I did not remove it at all, the following message appears:
The file could not be written to disk.

I have no idea what is going on since I did not change the code at all, and I did not do any changes on the web server (FreeBSD Platform) as well.

Any other suggestion to solve the problem?

Thank you in advance.