After a long time of searching, I found a possible reason why many users are getting a common mailtodisk error, even after configuring php.ini
The error is caused by unused system drives Such as the ones used by an SD card reader.
Fixing (Win 7, among other versions)
1. Go to Administrative tools select Computer Management > Storage > Disk Management
2.Find unused disks (These are usually marked with removable, along with no media. Typically constitutes an unused SD card slot)
3. Disable disk by Right clicking on Drive > Properties > Driver > Disable Accept warning message.
4. Repeat for all unused SD card slots.
>Take care in not disabling drives you currently use (e.g. CD drive)
>>Remember to re-enable these whenever you want to use SD Cards by going to Device Manager, find all drives marked with a down arrow icon and enable them
Normally fitting an SD card in the slot causing error should fix the problem, but only while a card is present.
Best of luck, I hope you found this helpful. Also please tell if this post helped at all!