Not on a Mac...-Your computer gets a virus which requires you to replace it
But that's not a good thing to rely on. It's expensive, and a large HDD for backups is cheap.-Your computer just plain breaks, so the data can't be accessed (well, it technically can)
+1 always back up!!!!!ssds can die just like any other drive. Data corruption can still be a problem. You lose the issue of long term mechanical wear, but these still carry limited write cycles, and your notebook could still be lost or stolen. Your data security is no greater with an ssd than an hdd for all practical purposes, so if it was worth backing up before, it is worth backing up now. You may never experience an issue, but many people have never experienced a dead hard drive. If it works for years, they get complacent.