Dual booting issues on the Apple Mac

Time Machine on the Apple Mac is a very useful feature if you have a spare USB disk drive. It makes backups in the background and it runs every hour, so you will never lose any of your valuable files. However, I came across something really odd the other day.

I have a new Macbook and I got it a couple of weeks before Mountain Lion came out, so it had Lion on the hard disk. Later I created a new partition and installed Mountain Lion on it so I have both versions of OS X. The disk is quite big at 500Gb, so two 250Gb partitions is fine for me.

This has had some unexpected effects. I expected the two partitions to be completely separate as they are in Windows. You can install two versions of Windows on a PC in two disk partitions and use them independently. In some respects though, OS X doesn't treat the two versions as separate and both partitions are used. It treats the other partition as part of the system.

Here is an example. I installed some software on Lion on partition 1. If I boot up with Mountain Lion on partition 2 and right click a file to open it in an application, apps that are installed in Lion on the other partition are on the list. I can open a file in an application that isn't even installed in Mountain Lion and it runs from the other partition. If I have installed a newer version of the app on Mountain Lion on partition 2 it offers me the choice of running the new version of the app on the Mountain Lion partition or the older one on the Lion partition. That's interesting.

One thing that really confused me is that I tried to set up Time Machine for the first time on Mountain Lion. It's new, so I have only used 52Gb of the disk. I had a free 128Mb partition on a USB drive that I thought I would use for the backup. However, Time Machine wouldn't start and it just displayed a message saying that there wasn't enough room. How can 52Gb on the Mac's disk not fit into a 128Gb USB drive? After a while it occurred to me that it wanted to back up Lion on partition 1 too, because adding up the disk usage of both partitions matched the disk space it said it needed. On giving it a larger 400Gb partition on the USB drive to back up to, it ran fine.

This is interesting, but it also raises the question of whether I need to run Time Machine in Lion as well as Mountain Lion, wether they should both use the same backup disk, and whether both are compatible. Will Lion Time Machine update or corrupt a Mountain Lion Time Machine backup?

I don't need to backup that often (I wish Time Machine was more configurable and you could make daily backups instead of hourly ones), so I think I'll switch off Time Machine in Lion. Backups will be made when I run Mountain Lion. Another option is to click the Options button in each Time Machine and exclude the other partition from backups. Then each Time Machine would only back up its own partition. I think switching off Lion Time Machine is the best option for me.