BootCamp – How to Install Windows 7 via USB

With the advent of Macs that have no optical drive, there's more information on this now:

http://howto.cnet.com/8301-11310_39-20020513-285/install-win-7-on-macbook-air-from-a-usb-drive/

You've trashed your partition table. Here's how:

1. Windows/OS X dual-boot setups almost always use a hybrid MBR, which is a dangerous (as you've discovered) hack that involves copying up to three entries from the GUID Partition Table (GPT) into a Master Boot Record (MBR) partition table. This creates the opportunity for the GPT and MBR data to go out of sync.
2. When you resized your Mac partition (presumably in OS X), the Mac tool adjusted the GPT side, and probably also the MBR side. So far so good.
3. When you resized your Windows partition in Windows, you adjusted the MBR side without touching the GPT data. Since the GPT is the real partition table, this means that your partition table was now damaged, although not irreparably.
4. When you used gptsync from rEFIt, it created a fresh hybrid MBR, basing it off of the GPT data. Since the MBR data contained the only accurate record of your Windows partition, this effectively trashed that partition.

How bad this is depends on precisely how you resized your partitions. If your Windows partition was first on the disk, it might be relatively easy to recover the situation, since you should now have a Windows partition definition with the correct start point but an incorrect end point. This might be relatively easy for some utilities to recover; however, given your symptoms and the usual way these things are laid out on Macs, my suspicion is that your Windows installation came after your OS X installation. In that case, when you resized Windows, you moved its start point. If this is what happened, then the steps you took mean that the new Windows partition start point is now lost.

In either case, your best hope of recovery is to use a tool like TestDisk, which is a tool for identifying "lost" filesystems. If this recovery is successful, you'll end up with a correct GPT holding all your partitions, including your resized Windows partition. You'll then be able to use gptsync to create a fresh hybrid MBR, and with any luck Windows will start booting again. Ironically, you may need to delete what appears to be your Windows partition for the recovery to succeed. (I recommend using gdisk for this task, since I can guarantee that it won't touch what's inside the partition. I'm not sure what Disk Utility does when it deletes partitions.) Note that the TestDisk recovery procedure is not 100% risk-free, though, so I strongly recommend you create a complete backup of your OS X installation before proceeding. If this fails, you could try looking for another tool that does the same job; it's conceivable that one tool will work when another doesn't. If you can't recover in this way, you'll just have to re-install Windows from scratch.

In the future, remember rule #1 when dealing with hybrid MBRs:

ALWAYS MODIFY PARTITIONS WITH GPT-AWARE TOOLS!

Using the GPT-unaware tools in Windows to resize the Windows partition is what got you into trouble, although gptsync's blind acceptance of the GPT data in the face of conflicting MBR data also exacerbated the problem.

You can also thank Apple for this problem; they're the ones who pushed hybrid MBRs on their customers, despite the fact that they violate the GPT specification and are an accident waiting to happen. (You're far from the first person to be bitten by this problem!)