Time Machine/Daisy Chain FW/FW & USB

[roland.g]

I understand that Time Machine can back up from an internal + external(s) to another external.

I have a 750 internal and a 750 internal and am looking to offload my media to the external and then get a 2TB external to Time Machine both drives to.

My 750 external is FW800. Am I better off getting a 2TB USB 2.0 or a 2TB FW800 as the Time Machine drive. I didn't know if the daisy chain aspect of 2 drives and TM reading one and trying to write to the other is a potential bottleneck/data conflict issue or if it is better to read from the FW800 drive and write to a USB drive. The USB solution is cheaper. But I was curious about the technical aspects of it mostly.

 

[Furrybeagle]

I’m not exactly sure how backing up from one external FW drive to another works, but look at it this way: when Time Machine backs up the internal drive, you’d supposedly use the full 800 Mbit/s speed of the interface to write to the Time Machine drive. When backing up from one external drive to the other, one would think the worst case speed would be ~400 Mbit/s reading from one drive and ~400 Mbit/s writing to the other. Compare to USB, which would use 480 Mbit/s for both drives.

However, I have no factual data to back up this up. Perhaps the performance hit from writing and reading on the same Firewire bus is bigger than just “splitting” the bandwidth into two.

Daisychaining also has the convenience of just plugging in one cable and using the USB port for something else.

 

[roland.g]

Thanks for the reply. I talked to a IT guy who had some insight and then called AppleCare and got a senior level tech.

For those who want to know:

First, Time Machine will back up from an external to your designated TM external, you just remove it from the 'exclude' list. However, you cannot restore using TM if that drive fails as TM will want to restore it to your internal not it's original location. However, as anyone who has used TM knows, you can browse the TM backup very easily, and copy the files to the new external if the drive fails.

Second, I was recommended to use a USB 2.0 not a FW800 daisy chained for the TM drive. What I came to understand is that in doing the backup, TM will copy the data up the FW cable to the logic board, the CPU will process it, and then it will be passed back down the cable. It would be better throughput and less potential conflict to have this going on 2 separate cables/ports because of bottleneck. You slow down the read/write process too much. Besides the USB is cheaper, and the backups are incremental and won't require that much time on the USB connection.

I am also toying with the idea of using my 2 750GB drives in a mirrored RAID for my media and getting a USB or FW drive for TMing everything on my internal. That may be the better route.