Quote Originally Posted by DanielBuus View Post
Question... This doesn't look valid either way. We're talking MBR, i.e. maximum four primary partitions sda1-4 OR three primary partitions (sda1-3) and an extended partitions (sda4) containing the remaining number of logical partitions (sda5...).
I got the impression that the disk was a hybrid MBR disk -- in other words, a GPT disk with a hacked MBR to enable Windows to boot from it. Linux doesn't require a hybrid MBR (it reads the GPT side just like OS X does), but many Linux installation procedures for Macs create hybrid MBRs anyhow, apparently to get the Mac's BIOS emulation mode to start up. This is confusing and dangerous, but its common.

In any event, if it's a hybrid MBR, then those are perfectly valid GPT partitions, but only three of them will be copied over into the MBR. (One MBR entry will be a type-0xEE partition to identify the disk as a GPT disk.)

That is, you can't have a partition containing an FS on sda4 if you also have sda5, as sda4 would then have to be the extended area containing the remaining logical partitions.
Even in a pure MBR setup, this isn't quite right. Any of the four primary partitions can be an extended partition, not just the fourth one. It's a bit less confusing if the extended partition is the fourth primary and if it (and therefore the logical partitions it contains) occupies the end of the disk, but neither of these is an actual requirement.

One other comment, since I began by describing hybrid MBRs: Hybrid MBRs should not contain extended or logical partitions. Although it's theoretically possible to create a layout like that, it would be an absolute nightmare to maintain. Fortunately, I don't know of any program that makes the attempt.