On a Mac, you don't need the smaller .mds file, just the .mdf one. Highlight the mdf extension and change it to ISO. The Mac will ask if you want to change to iso or keep mdf, select ISO. Now you can mount it with Disk Utility, burn it or copy files from it. If this doesn't work for you, then most likely the file is corrupt and a last ditch effort to get it to mount with Alcohol on a PC may or may not correct any errors.
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