SSDs lifespan is dictated by write access. In read only mode, they're virtually unbreakable. Write operation are numbered, though, and some manufacturers even specify how many writes you can do before breaking it.
Granted, we're not talking about tens, but rather thousands of writes.
Obviously, PI makes great use of the HD, with every image getting rewritten everytime it's aligned and/or calibrated. So that would be my first concern.
