By Graham K. Rogers

A few days ago I started the process of replacing my reliable 500 GB LaCie backup disk with a WD 2 TB My Book Studio disk, which should keep things rolling for a while.

Earlier versions of OS X Time Machine only allowed one disk at a time to be used, but with the latest version (10.8, Mountain Lion) users may allocate more than one disk for the process. This would be useful for off-site storage of backups, or in my case smoothing the transition from a disk that is full to its replacement.

I wrote about my preparations and what I did, when I made the changes. After a few days, however, I have noticed differences in the information displays and in the way the backup processing occurs.

I had the LaCie disk set up so that it turned on automatically when connected to the Mac. As I daisy-chained it via the WD disk, it now shows On even when the Firewire cable from the WD disk is not attached to the computer. This is not a problem, but just a note on the changes.

Unlike earlier versions of the disk, as reviewed by sources like MacWorld, there is no display panel on the front of the WD disk housing, nor is there any On/Off switch. Only when data is being transferred does the small light on the front work.

As I have the disk facing the wall, to give me easier access to the ports, most of the time I do not see this (the LaCie disk is slight taller and the light reflects in a mirror). I leave this to trust and to the occasional look at the Time Machine preferences panel.

The display in System Preferences has changed significantly. In the past, when the single disk was not attached, text would indicate that the next backup would occur, when disk is attached (check that). When the disk was attached, the panel would show the time that the next backup was due. With two disks now nominated, smaller icons for these are shown with a third Time Machine icon just below for adding or removing disks. There is no way to adjust the height of the pane.

Disk information now shows the date and time of the first and the most recent backups for each disk. When I added the new disk to the list, Time Machine told me that backups would be made alternately. I see that this is the case.

When I put the cable into the FW800 port, the disks appear on the desktop but only the LaCie disk shows a Time Machine icon. The second partition on that disk is shown with the FireWire icon as is the WD disk, despite this being shown in the Time Machine panel as one of the backup disks. Again, this is another observation on behaviour with this two disk Time Machine set up.

Shortly after the daisy-chained disks are connected, the first backup begins. Which one depends on which was backed up last: the other disk is then used first. A second backup process occurs (using the other disk) some time later with this alternate processing continuing for as long as the disks are connected.

As before, there are two types of display for the progress bar. Initially, a barber's pole type may be seen with a text comment that the system is "preparing" for backup. This may also show that old backups are being deleted. As the backup process begins, the bar changes to a solid aquamarine blue that moves across the panel as the backup is made. A text display also shows how many MB or GB are being transferred.

On the whole, the first stage of the transition to a new Time Machine backup disk has been straightforward. In a week or so, I will click on that other icon, below the current disks and select Remove.

Graham K. Rogers teaches at the Faculty of Engineering, Mahidol University in Thailand. He wrote in the Bangkok Post, Database supplement on IT subjects. For the last seven years of Database he wrote a column on Apple and Macs.