Being a Mac tech for many years I have always had a “Tech Drive”. You can keep all kinds of different things on a Tech Drive including Mac OS updates and other software. My Tech Drive has a tech partition and several other partitions with various Mac OS installs like Snow Leopard, Lion, Mavericks, etc.
I pretty much use my Tech Drive in two ways, both ways involve booting a Mac from it. It connects via USB. I either boot from the Tech Drive and install a Mac OS from one of the partitions or I boot from it and go into my Tech area. Here is a photo of the bootup using my iPhone (just click on it for a larger image so you can read the text).
The Tech area is a bootable partition which is running the Mac OS. The Tech partition contains several utility programs for fixing Macs like Disk Warrior, Drive Genius, Data Rescue, etc. These are great Mac utility apps that have been around for many years that are used for repairing the Mac OS, checking hard drive functionality, rescuing data from dying hard drives, etc.
One of the keys to keeping the Tech partition viable is to keep the OS updated on it. That way it can boot the latest machines. So, recently, I updated the OS in the Tech partition of my drive. It was running Mavericks 10.9 and I wanted to put it on Yosemite OS 10.10. But instead of downloading Yosemite from Apple I copied the Yosemite installer from my iMac over. This saved me about 20-30 minutes:
Once it was copied over to the Tech partition I ran the installer. It installed just like on a normal hard drive:
Once it was installed and was booted into the Tech partition I updated it to 10.10.1. Believe me, the total elapsed time compared to downloading from Apple and then installing was quite a bit less.
Now, I should have no problem booting newer machines from this Tech drive. Even though I do not work in the Tech support area anymore, I still work on mine and my friends and families Macs. The Tech drive comes in real handy.
Just an aside, if you wanted to have a Tech partition for an older PowerPC machine you would have to have an older OS installed on it. I think the highest you could go would be OS 10.6, Snow Leopard.
If you are at all inclined to work on your’s or your friends Macs it is not too difficult to make a Tech Drive. I just used a cheap 2.5” laptop drive from Amazon and put it in an enclosure I have. It is USB 2.0, but it does not have to be fast to work. Just install a Mac OS on the Tech Partition and you are off and running.