Some of my spell checkers (including the one in vim, and the build-in one in gmail) mark the form "copiable" as an error, and insist on "copyable". This doesn't seem to make sense; the rule I always learned was that "y become i if preceded by a consonant when adding a suffix, unless the suffix starts with an i: eg "copied" or "copier", but "copying". According to this rule, the correct spelling would be "copiable", which is what I always use. But I often see "copyable", which doesn't seem right to me. (For what it's worth: Merriam Webster says that the word doesn't exist, in either form, the American Heritage Dictionary gives both forms, and the Oxford English Dictionary only gives "copiable".)
What is the correct usage, and why?