I know you asked Mir911, but after matching up the MS...

I know you asked Mir911, but after matching up the MS announcement with the transactions reported in the Athos document, it seems Athos have been behind the MS holding all along (although I don't see how this is legal, to be honest).
MS still holds several million shares by my calculation.However, it puts to bed the idea that someone (eg. Mitsui) is using TPD to acquire a small stake in STX (sorry guys as this was my speculation), at least, via Morgan Stanley's reported holdings.
Also, after a bit of digging, it turns out Athos fund emerged from Capita Venture Partners, who are on record as using a trading strategy that involves shorting an acquiring company and buying the target company. It also aligns with the name of the fund reported by Athos:

This also aligns with what SN said about an "arbitrage" event (as well as Gimo, Rexsh and others). Although, I would say this is not really arbitrage (i.e. risk-free trading of assets), rather a trading strategy with some inherent risk. For example, if the SOA gets voted down, Athos won't have the STX shares to repay their borrowed STX stock, and in the meantime, if STX goes up, they will get squeezed. But the risk of SOA being rejected is small, I think.