What word am I looking for? Related to unique parts or non-universal components

For example. You cannot use Mac parts in a Windows computer because they are ________ parts. <- I believe the word I am looking for starts with an S.

Unique, not universal, can only be used on a specific platform etc..

any help would be great thanks!

Solution 1:

perhaps proprietary?

1:  one that possesses, owns, or holds exclusive right to something; specifically :  
    proprietor 1
2:  something that is used, produced, or marketed under exclusive legal right of the 
    inventor or maker; specifically :  a drug (as a patent medicine) that is protected by 
    secrecy, patent, or copyright against free competition as to name, product,
    composition, or process of manufacture
3:  a business secretly owned by and run as a cover for an intelligence organization

This word is often used to refer to this sort of situation, where no-one else is allowed to manufacture the part in question as the technology is wholly owned by the proprietor (Apple in your example), and because the technology is product-specific it (usually by design) won't work with another product (such as a PC).

Solution 2:

incompatible

Definition:

1.3 (Of equipment, computer programs, etc.) not capable of being used in combination:

"all four prototype camcorders used special tapes and were incompatible with each other"

Your sentence would therefore be:

You cannot use Mac parts in a Windows computer because they are incompatible parts.

This is not a word starting with "s", but I think it's the most relevant, appropriate and commonly used. I couldn't find any appropriate synonym that met the "s-word" constraint.

Synonyms include:

unsuited, mismatched, ill-matched; ill-assorted; irreconcilable, conflicting, opposed, opposite, contradictory, antagonistic, antipathetic; clashing, inharmonious, discordant