Long story short, if you want to add a second hard drive to a T60, T61, T61p, R60, R61, Z60, Z61, etc., then you want to buy an OEM caddy. They sell knockoffs on eBay for $20 that I assumed would work "just fine," and well, let me tell you -- I got burned hard. Lost a ton of data that I hadn't backed up, and didn't even realize there was a problem until it was way too late because of the way the data corruption showed up. According to a couple poster over at NotebookReview, this is pretty commonplace with the knockoff drive caddies. Would have been nice to know!
In any case, I obviously replaced that caddy with a real Lenovo OEM one. Wasn't cheap, but works flawlessly and was completely worth it. As luck would have it, I ended up selling my T61 about two weeks after ordering this caddy, so it needs a home.
I paid right at $67 shipped for it, so I'm looking for
$45.00 shipped which seems quite fair -- it's less than the OEM ones go for on eBay, and I encourage you to take a look.

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This is definitely the way to go with your laptop. Nothing sweeter than running your OS off of an Intel X25-M with a 750 gig hard drive riding shotgun to hold your data. Thanks for looking!