Thinkpad T420 notes

Will Windows 7 boot if the C: partition is made active?

Yes.

A 1,200 MiB  NTFS partition in MBR entry 1 was active.

Will Windows 7 boot if the 1,200 MiB partition is deleted?

Yes.

What will then happen if the ThinkVantage key is pressed during boot?

A blinking cursor in the top left corner of the screen.

Will Ctrl Alt Del work? Yes.

Well, the three last steps were probably with a custom Lenovo MBR.  With a standard MBR, the "Startup Interrupt Menu" will appear if the ThinkVantage key is pressed.

Can we boot from an SDHC card? No, the card is not visible until operating system drivers are loaded.

It seems as the card reader may not work in Windows 7 if a harddisk is inserted in the Thinkpad 12.7 mm Serial ATA Hard Drive Bay Adapter III. P/N: 0A65623. It works in Linux. - No, that was not the explanation. Again the card was not seen in Windows.

The "One Key Recovery" option requires that some Lenovo custom program code or data is present between the MBR and the first partition. This for example will be overwritten if the Grub boot loader is installed. One way to restore the content may be this: Make a copy of the \Boot\BCD file on the first  (SYSTEM_DRV) partition. The partition normally is not visible in Windows, and the files and directories are hidden, but the file can be copied for example from boot to Windows PE.

Then install the Lenovo Rescue and Recovery program on the Thinkpad. That will rewrite the custom MBR and the custom sectors after the MBR. Then copy back the \Boot\BCD file to the first (SYSTEM_DRV) partition. The features of Rescue and Recovery when pressing the ThinkVantage key are then not available, but the original ThinkVantage key behavior may be restored.

Make a backup copy of the current BCD  file before you copy the old BCD file back.

I am not quite certain that this is correct, so only do it if you have backup, and know what you are doing.