Remember to sync (or what the heck is that RAW C: partition on my Windows on mac?)

Our current computers use MBR to store the table of primary partitions. MBR for convention allows only 4 primary partitions and then uses Logical Partitions inside Extended Boot Records for the successive. Instead Apple computers use the new Extensible Firmware Interface with the GUID Partition Table to describe disk partitions permitting any number of primary partitions to be allocated.

MacOSX uses the GPT partition table while Windows expects to find the MBR one (Actually Windows for Itanium can boot from GPT, but that is not what you are usually installing on your desktop). Fortunately the EFI keeps a “fake” MBR table for backward compatibility and that is why we can install Windows on our MacIntel. But what happens when we change our GPT partitions from OSX/Linux which supports GPT? It happens that our MBR becames out of sync with the GPT.

Recently I had to install a triple boot with OSX+Win+Linux on my MacBookPro. This required 5 partitions (1 for EFI, 1 for OSX, 1 for Linux, 1 for Windows, 1 Linux Swap). After creating them from OSX I synchronized the MBR to GPT with refit tools to have the first four partition available also on MBR (this way I lost only the SWAP partition)  then I installed Windows XP. Everything went well but after booting, Windows ended up installing on D: and there was a C: partition resulting of RAW type. I just thought that obviously this was because there was the un-formatted Linux partition before the Windows one and restarted to install Linux.

Then I formatted my linux partition as EXT3 and installed Gentoo over it. Everything was fine until I had to restart windows to install the Service Pack 3 for XP. My wonderful RAW C: partition was still there… and I wasn’t able to install the service pack because SP3 installer wasn’t able to install on anything else then C: even if my Windows installation resulted on D: and C: didn’t result as a valid partition (off course… It’s my Linux partition!)

Windows should have known that it isn’t a valid partition for him and should hide it, that didn’t happen… Why? After thinking a bit I remembered that the Partition Type is also a field inside the MBR. When we format a partition the system specifies also the partition type inside the MBR to tell everyone that partition has been reserved by some OS. Having initialized my EXT3 partition from Linux had as a result that only the GPT was updated and the MBR still thought that my third partition was un-formatted.

After losing about an hour fighting with the Service Pack installer I just realized that I had to resynchronize the two tables… After doing that the RAW C: partition disappeared from Windows and my service pack installed correctly.

New Lesson: Ok, Microsoft could make a better installer and stop preventing their OS to install from something different from what they call “C:” but sometimes you have to think more before enraging with your OS :D

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