Windows 7: Can't add drive letter to partition, have 7 primary partitions

Ok i know this sounds stupid (hey, it's windows) but how do I end up with 7 primary partitions? I thought you could only have 4. Anyway, one of my partitions no longer has a drive letter so I can't access anything on it. I'm unable to right click it in disk Management to add one, the option is grayed out (see pic).



1st partition = 100MB Windows 7 boot
2nd partition = the windows OS on it's own 50G partition
3rd partition = 50G FreeBSD Unix (this is a multi-boot system)
4th partition = 500G data partition, the one that lost it's drive letter D:.
5th, 6th, and 7th partitions are Linux partitions and the last one looks like an extended partition by the green outline.

I've used this system successfully since last year when Windows 7 came out. It's worked pretty good but all of a sudden it lost the drive letter for the 500G data partition. I have no idea what happened or how to get it back. Any ideas?

Shot in the dark:

As you said, you can only have 4 Primary Partitions. I suspect that Windows knows this and is therefore saying "Well that can't be right" and assuming its made a mistake and won't let you assign drive letters.

If you get it down to 4 partitions again, it will probably let you assign letters.