Re: Is Triple-booting with Ubuntu in middle possible?
One caveat is that Windows relies on a hybrid MBR to boot on an Intel-based Mac. As described on the page to which I linked, hybrid MBRs are a dangerous hack, and if you do things wrong, you can seriously mess up your computer. Most importantly, you should do your partition resizing from OS X's Disk Utility or some other tool that understands the GUID Partition Table (GPT) partitioning scheme that Intel-based Macs use natively. If you use Windows, it will see the disk as MBR, resize the MBR representations of the partitions, and leave the GPT representations alone, which will create a dangerous inconsistency. I realize you probably wouldn't use a Windows tool to shrink your OS X partition, but I want to point out this danger since I've seen posts and even gotten e-mails from people who've seriously damaged their disks because they used Windows partitioning tools on disks with hybrid MBR configurations.
A less serious issue, but one that could easily become a stumbling block, is that when you finish installing Ubuntu, you'll probably have a GPT-only disk rather than a hybrid MBR. Various tools will enable you to create a fresh hybrid MBR, but most of these just add the first three partitions (aside from the EFI System Partition, or ESP, which OS X's Disk Utility hides from view) to the MBR side. Depending on how many partitions you use, putting Windows at the end can result in Windows being left out of the MBR, which means that Windows won't work. This problem is easily overcome by a tool that gives you more control over your hybrid MBR creation. I've seen patched versions of gptsync that do this, but these patches don't seem to have caught on. My own GPT fdisk (gdisk) program can do the job, too -- see the earlier hybrid MBR link for details of how to use it to create a hybrid MBR.
One more point, which is a general multi-booting issue: I strongly recommend creating a separate FAT or NTFS data-exchange partition. It's generally unwise to access any OS's boot partition from another OS, since non-native filesystem support is often imperfect and security features are often useless in other OSes. These limitations make it easy to accidentally trash a boot partition. Using a separate data-exchange partition minimizes these risks and isolates them to a relatively unimportant partition. Such a partition should, of course, be included in the hybrid MBR along with the Windows boot partition.
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