As long as the path is valid, that should work (with or without providing the drive letter). /copy will not create a folder that does not exist. Also, if you're using spaces at all in the path or filename, you'll run into problems unless you put quotes around each path/filename.
Although you should be able to use / in the path, with Windows, it really should be \. / is used in *nix environments for paths, but Windows uses \ except in regard to network paths. Although mIRC should work fine with /, you might want to try with \ and see what happens.
One other possibility for the error is that the file is locked for some reason. It may be set to read only or you may not have permissions to modify/delete the file. It may also be "in-use" such that it cannot be modified.