You can probably discard the PHP warning "should be compatible"... This is a well known warning and it is usually harmless. If you want to fix it however, see this post by Andreas Gohr himself: https://www.patreon.com/posts/declaration-be-20638123
Now, the error message "PHP message: PHP Warning: Unknown: POST data can't be buffered; all data discarded in Unknown on line 0".
Well... data probably too big. Why is hard to say. I don't think you're hitting some sort of PHP or apache/nginx/lighty/whatever limit. This sounds like "Hey, I was trying to write the upload in a temporary file but some unexpected situation happent. I give up".
So I'd go for a disk problem. A "disk full" error or some other problem with the disk that prevents php from storing the temporary file. Note that the disk may not be /mnt/hd1 and could be your system disk. Check the value of upload_tmp_dir
php.ini directive to find out which folder is used for temporary uploaded files. If it's empty, php will use some system defaults that most probably aren't anywhere under /mnt/hd1.