Update: and now Geoff Baker says this isn't true, and that Garko could've been optioned at any time. You know what? I don't care. I don't even care. This story is too confusing and I'm done with it.
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As suggested by G_ in the comments and confirmed by Dave, there's a rule in place stating that, if it's been more than three years since a player's debut, he must be passed through waivers before being optioned. Ryan Garko debuted in late 2005.
It's a rule that doesn't come up very often, but it's a rule nonetheless. Here's the good news: the waivers are revocable, and this process is considered something of a formality, as teams seem to have an agreement not to claim other teams' players. As one example, Brandon McCarthy just cleared a few days ago, and he's a starter with decent promise. So, even though Garko's been waived, the odds are that he clears, at which point the M's could either option or outright him to Tacoma. In the event that he's optioned, another spot would have to be opened up on the 40-man. They could also just dump him, I guess, if they're that unimpressed, but, ehhh
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I still like Garko more than Sweeney.