Disastrous has three syllables but, yes, an extra syllable is often added in speech due to the relationship between this word and the word disaster.
Neither it is an added syllable
prefix
prefix
prefix
A prefix is a syllable that is added to the beginning of a word to change its meaning or create a new word.
The syllable added to the end of a word is called a suffix. It can modify the meaning or change the grammatical function of the word.
prefix
Suffixes are things added to the ends of words, although they can be more than one syllable long.
The syllable added at the beginning of a word is typically called a prefix. Prefixes are added to the beginning of a word to change its meaning or create a new word.
A suffix.
I believe you're referring to a prefix.
A suffix.