Issue: You downloaded a file from Microsoft, such as the Windows 10 Preview. The ISO downloads successfully, but when you try to mount it using Windows 8 or 8.1, you get the error:
Sorry, there was a problem mounting the file.
Cause: Sometimes when you download a file from Microsoft it has the sparse flag set on it, which prevents it from mounting the ISO.
Fix: Remove the sparse flag.
You can do this two ways:
1) Copy the file. The copy removes the sparse flag, so your copy will work; however, since ISO files may be gigs of data, this may not be an option.
2) Remove the flag using the fsutil command.
fsutil sparse setflag yourfilename.iso 0