Is it not possible to have a bootable partition on an external HDD?

[ideal.dreams]

I have a 1TB external hard drive I currently have legacy Time Machine backups on from my old MacBook. I want to partition it so I can install a copy of OS X Yosemite on it however disk utility isn't allowing me to use the options button after partitioning to format the disk to GUID structure.

Is it not possible to achieve this or am I missing something?

 

[simsaladimbamba]

You are in the PARTITION tab and have selected "1 partition" or similar from the download menu and are unable to click the OPTIONS button because it is greyed out?

How to format a new HDD to install Mac OS X onto (or make a bootable copy onto)​

 

[ideal.dreams]

You are in the PARTITION tab and have selected "1 partition" or similar from the download menu and are unable to click the OPTIONS button because it is greyed out?

How to format a new HDD to install Mac OS X onto (or make a bootable copy onto)​

No, because I don't want to overwrite the data that is currently on the drive. At the moment, the drive has only one partition with old Time Machine backups on it. I'm trying to add a partition to the same drive so that the data remains intact but I will also be able to boot from the new partition.

 

[DeltaMac]

I don't think you can adjust the partition scheme for one partition, if the rest of the drive is something else.
What is the partition scheme for the device now? (select the line in Disk Utility for the manufacturer's info. Should show you the partition scheme in the information block.
If it is already GUID, you should be OK. Just erase your added partition, making it Mac OS Extended (journaled)

 

[Fishrrman]

This probably isn't going to work.

I've found that with later versions of OS X, for a partition to be bootable, it must reside towards "the beginning" of the drive (physically, insofar as the sectors are concerned).

If you want a bootable partition on an external drive, my guess is that you'll need to reinitialize the drive, then create two or more partitions on it, and then install yosemite onto the FIRST partition (highest one) as illustrated in the Disk Utility window.

Not sure why this is so, but I've seen it to be true on my backup drives.
BTW, I don't use Time Machine, all the backups are created using CarbonCopyCloner.

Aside:
I wouldn't put Yosemite on a drive that contained important backups in any case.
I'd put it ONLY onto a "standalone" drive that had nothing else on it, and that I could "wipe clean" without fear of losing anything...