How to boot external with multiple partitions?

Hi baltwo! Thx for your reply!

Did I get you right? You basicly said, that every partition should be poping up in the startup manager, right?

Well, that exactly is the problem ... they do not :-( To be more concret: The external 3TB drive has 6 partitions in the following order. The lower case "b" means bootable:

PartAb, PartB, PartCb, PartDb, PartEb, PartF

Starting up the MAchbook Air with Option key pressed, runs into the Startup Manager as expected. And the Startup Manager offers the folloing:

Macintosh HD, Recovery, PartAb, PartCb, PartDb

Just as expected, the not-bootable Partitions are not offered. But as not expected the PartEb is missing. It looks like the number of offered Drives to boot from is limited somehow.

All bootable Partitions are created the same way.

Do you have any idea?

Thx for your help!

Thx for your reply, but I dont think that "bless" would be of help here. It may can be used as a workaround but actually does not address the issue of this thread.

In case my internal "Macinbtosh HD" is on failure and I need to boot from my external partition, I do not have a command line to "bless" the partition of desire (PartDb). All I have is the startup manager which does not offer partition PartDb. The command "bless" may would work if I boot from any external partition which the startup manager offers (e.g. PartCb) and use the command line of this OSX just and only to "bless" the partition I actually need, the PartDb. In case this works I would need to perform a subsequent backup for the computer using PartCb as its backUp is not anymore fully correct.

Thx again!

Thx for your post baltwo!

I noticed, that I have made a typo in my post right "above" yours. The partition of desire is, of cause, PartEb and not PartDb. While PartDb is offered from the startup manager, PartEb is not.

However, its unlikely that something is wrong with PartEb. All Partitions are created and filled the same way.

I will do some testing with bless and different MACs and see whether I can track down this issue a little more,

Yep, I did some tests and finally got it solved.

I re-arranged the order of partitions and put all 4 bootable partitons in a row. Now they are all offered in the startup manager.

It did not help to just redo the missing partition. May something was wrong in the meta information on the disk about the partitions.

Thx nbar and baltwo for your thoughts on that issue!

Just another add on - to be fully complete ...

While deleteing and recreating the missing partition in order to place it at a different position, I resized it from former being 70GB (BackUp for a small 64GB MacBook Air) to 150GB to have spare room. I might will upgrade the MBA soon ...

I do not have any idea why and whether this contributed to the solution, but when somebody in future will ran into the same problem he/she should be aware of all chenges I made ...