The setup you describe can be done and is even supported by Microsoft. But it must be done in a very specific way or it won't work. Microsoft has published an article on exactly how to do it (Note: this is a hot download link). It applies to Server 2008 but it applies equally to Windows 7.
To summarize, there are two issues at play here that you did wrong when you set things up:
Creating a mirrored volume requires you to convert both disks to dynamic. The BIOS of your computer does not understand dynamic disks. It simply boots whichever partition is marked "active" in the partition table. To solve this problem, Windows created a "fake" partition table when you did the conversion on the primary drive so that the BIOS would still see it and know how to boot. But I'm betting that when you added that second drive, it had no partitions on it (or you removed them) before you converted it. That fake BIOS partition table only gets created if a real, active, bootable partition exists WHEN THE DRIVE IS CONVERTED to a dynamic disk. If a drive has no partitions, the fake partition table does not get created, and there's no way to create one after the fact. The secondary drive therefore cannot be booted by the BIOS.
With Microsoft, mirroring works at the volume level, not the disk level. The default setup for Windows 7 is to create a 100MB reserved partition that gets no drive letter, and the remaining space becomes drive C. Your Boot Configuration Data (BCD) store lives on that reserved partition. When you added the second drive you probably didn't create that reserved partition, so a BCD store does not exist on it. Even if you do create that partition, it cannot be mirrored. You must clone the contents manually to the second drive, and manually sync them again if your BCD store ever changes (fortunately that's rare).
If you read that document I linked to above carefully, you'll notice that they have you creating partitions on the 2nd drive and cloning the boot files before the step where you convert it to dynamic. They also have you manually adjusting the BCD store on the secondary drive as well (because if you boot off it, it's not the secondary anymore -- the first drive is).
Oh, and one more thing I should mention:
You do not gain any speed advantage when using a software RAID-1 in Windows. A real RAID controller treats both drives as equals and does reads and writes to both drives simultaneously. Windows' mirroring will ALWAYS do reads from whichever disk you booted from. The secondary drive is simply kept in sync but otherwise not used for anything. The feature only exists for fault tolerance so that a server can continue running if one of the drives fails. It's kind of pointless on a workstation in most cases.