Gnu:

The argument is not about the noun, disk. It is about the initialism, CD.

"Use in forming certain plurals

An apostrophe is used by some writers to form a plural for abbreviations, acronyms, and symbols where adding just s rather than 's may leave things ambiguous or inelegant. Some specific cases:

    It is generally acceptable to use apostrophes to show plurals of single lower-case letters, such as be sure to dot your i's and cross your t's. Some style guides would prefer to use a change of font: dot your is and cross your ts.[citation needed] Some style guides rule that upper case letters need no apostrophe (I got three As in my exams[44]) except when there is a risk of misreading, such as at the start of a sentence: A's are the highest marks achievable in these exams.
    For groups of years, the apostrophe at the end may be regarded as unnecessary, since there is no possibility of misreading. For this reason, some style guides prefer 1960s to 1960's[44] (although the latter is noted by at least one source as acceptable in American usage),[45] and 90s or '90s to 90's or '90's.
    The apostrophe is sometimes used in forming the plural of numbers (for example, 1000's of years); however, as with groups of years, it is unnecessary because there is no possibility of misreading.[citation needed] Most sources are against this usage.[citation needed]
    The apostrophe is often used in plurals of symbols. Again, since there can be no misreading, this is often regarded as incorrect.[44] That page has too many &s and #s on it.[citation needed]
    Finally, a few sources accept its use in an alternative spelling of the plurals of a very few short words, such as do, ex, yes, no, which become do's, ex's, etc.[46] In each case, dos, exes, yeses (or yesses) and noes are preferred by many authorities.[citation needed] Nevertheless, many writers are still inclined to use such an apostrophe when the word is thought to look awkward or unusual without one."

http://en.wikipedia.org/wiki/Apostrophe#Use_in_forming_certain_plurals