Thanks for everyone posting to this thread, I've been wanting a second hard drive for my X301 for quite some time now.
Based on what you guys said, I ordered this: http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... 0914686538
and this: http://www.newegg.com/Product/Product.a ... 6822145530
I got those yesterday and installed them but in my case Windows 7 32-bit didn't boot with the caddy + hard drive in the UltraBay until I changed to IDE mode. Windows startup repair (press F8 at boot) wasn't able to fix the change from AHCI to IDE for some reason so I had to do it manually. Some/all of these steps may/may not apply to you, depending on your setup.
1) Turned on the laptop without the caddy in the UltraBay
2) enabled IDE in Windows Registry by setting the value for "Start" to "0" in HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Pciide
3) rebooted and changed the BIOS setting for SATA from AHCI to Compatibility
4) rebooted and let Windows install the "new" IDE hard drive (=my Toshiba 128 GB SSD)
5) Shut down the laptop, inserted the caddy + drive in UltraBay and rebooted to Windows
6) Windows detected and installed the drive. Rebooted once more just for fun.
Depending on your current setup the caddy might work just fine without any of those steps. I guess my problem was the lack of IDE devices/drivers (I always used the travel cover in the UltraBay: http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... 0521459188
) and hence the IDE drivers were not enabled. I tried re-enabling AHCI after step #6 but Windows didn't boot again, even with AHCI enabled in Registry, so I guess I have to run my 128 GB Toshiba SSD in Compatibility/IDE mode from now on.
The seller Nimitz shipped the caddy promptly and it arrived from China to Seattle, WA in only nine days, despite the New Year holidays in between. He also answered my question in under 24 hours. The product itself appears to be of high quality and fits well in the slot. As of writing this I've had it running only a couple of hours but it seems to work fine and writing dummy random data to it with TrueCrypt shows a consistent 70 MB/s write speed to my non-SSD hard drive.
I highly recommend using the bottom screw to hold the caddy in place. Happy 2012 to everyone.
Thanks, TXFI! I recently got my hands on an x301 in great condition with a 64GB SSD and wanted to add a larger HDD in the Ultrabay. I bought a caddy similar to the one mentioned here and installed a 500GB Toshiba HDD, but the Thinkpad wouldn't recognize it. I edited the registry and changed the BIOS setting according to your post, and now it works perfectly. Thank you!