I know this dredges up an old post but I had this error and thought I should post the solution I figured out. In my case the issue was that PHP was having problems writing a file to the temp directory and "Failed to write file to disk Error" was the result. This happend only for files larger than 1MB, even though all max upload file sizes were set to 100MB. My solution was to edit the php.ini file and make sure to set the upload_tmp_dir setting, make sure that folder existed and that write permissions were enabled.
Failed to write file to disk Error
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