Both .mdf and .mds files are used for Alcohol 120% disk images, but these two files are pairs that complement each other and not meant to be converted via mdf to mds conversion. MDF format works as the actual disk image, MDS files stores meta data about the physical medium (CD/DVD).
This conversion entry was verified by us and conversion was flagged as wrong or not possible.
Usually this happens when the conversion would try to convert different types of data between each other, such as audio to image, or it is not possible because incompatibility between programs, caused by closed file formats, very limited support etc.
If you find entry that you feel is flagged as bad wrongly, please tell us in the discussion below with as much as information as possible so we can fix it.