Is it possible to mirror 2 external hard drives?

Discussion in 'Mac Basics and Help' started by mfsnyc, Jul 23, 2008.

  1. mfsnyc macrumors newbie

    Jul 23, 2008
    I store all my photo files on an external hard drive (500 GB LaCie). Then I'd burn copies to DVDs as another backup ... but burning DVDS is extremely time consuming and I fear not a good solution for the future.

    I'd like to get a second external hard drive and have it mirrored to my first external hard drive. How could I set this up?

    I don't store my photos on my internal hard drive b/c it is too small - 80gb.

    I wonder if I should:

    1. Mirror 2 external hard drives (if possible, and if so, is this slow?)
    2. Install a new larger internal hard drive and mirror to an external hard drive

    or ???

    I've read about RAID set ups, Drobos, etc. and other posts on this site and am still a little confused as to what the best solution. I'm trying to keep it budget too.

    Thanks in advance.

    I have a 1.8 GHz PowerPC G5
  2. Fallinangel macrumors regular


    Dec 21, 2005
    SuperDuper! can do that!
    If you want to backup a backup, just tell it to copy one drive to another!
    It can even do smart backups that means that you won't have to copy all of the files everytime, but only the new and changed ones.
    SuperDuper! works with external and internal HDs, so there's no difference there.

    If I were you, I'd get an external drive with a little more space than your internal HD and use TimeMachine to do your backups!
    It's easy to use and secure (backups every hour).
    Also, the chances that two drives break at the same time is very, very, very small, so invest the money that you save on a third drive to get a better and bigger secondary drive.
  3. blodwyn macrumors 65816

    Jul 28, 2004
    Portland, Oregon
    You have a couple of options, depending on what your objective is...

    1. If you just want to guard against a drive failure, then a mirror (RAID1) is an option. The drawback to this is that it behaves like a single disk, so if you overwrite or delete a file, or have a filesystem problem, then your data is lost on both disks.
    2. If you want to guard against a drive failure, and have protection against the problems above, then replicating the drive is the way to go. This means you use one of the backup drives as the master, and periodically (daily, weekly or whatever) sync the master to the backup using Chronosync or similar. If you only update files on the master, and don't touch the backup drive, then you won't have any sync issues.

    I recommend option 2 in most cases. For reference, you can create a mirror'ed set up on 2 external drives, using disk utility. I wouldn't do this if you're using Tiger, as you have to reboot to re-sync the mirror if one drive is a bit slow waking from sleep mode, or if you unmount the mirror and try to remount it. Leopard is much better at handling this.

    Hope this helps
  4. mfsnyc thread starter macrumors newbie

    Jul 23, 2008
    Thank you both. I'm running Leopard 10.5.4. I'm after backup & concerned about drive failure. I want a backup solution dedicated to my photos. And ideally want to just buy one new hard drive.

    I think option 2 is what I want- I want 2 external drives duplicating the same info, but done automatically daily. I will look into Chronosync and SuperDuper to help me do this- is there anything built into Leopard OS that I have but don't know about that will do this? I don't think need constant versioning/archiving (as in Time Machine) as the files don't change much after my initial edit (and I'm using the backup solution on 2 external harddrives w/o OS installed on them).

    thanks again.
  5. Moof1904 macrumors 65816

    May 20, 2004
    Why not just use Disk Tools and set up the two external drives as a Raid 1 mirror or each other? It will only take a moment and seems like it would do what you want.
  6. blodwyn macrumors 65816

    Jul 28, 2004
    Portland, Oregon
    There are built-in options that you can put together using Automator workflows using a repeating iCal event to trigger, or using a shell script based on rsync (like this tip from macosxhints). If you don't mind spending the money, then Chronosync has proved very reliable and easy to use for me, and with some powerful options if you dig deeper into the advanced features. Just set the sync up and set the schedule to suit you, and sleep peacefully :)
  7. blodwyn macrumors 65816

    Jul 28, 2004
    Portland, Oregon
    A RAID1 mirror is fine protection against a mechanical drive failure, but does not do anything to protect against file system corruptions, inadvertently overwriting a file, mistakenly formatting the disk/ deleting the partition, etc (I'm sure if I checked, all the above will have been reported on this forum at some time).

    Having a replicated drive gives you the redundancy of a second copy of the files in case you screw up one copy, but with the slight overhead of maintaining the sync yourself. Automating the sync removes the overhead but introduces the possibility of replicating the screwed file to the backup if you don't notice it before the sync kicks in. Chronosync has the option to archive file deletions and replacements, which offsets this problem.
  8. mfsnyc thread starter macrumors newbie

    Jul 23, 2008
    Thanks again for your input. How much space does this Chronosync feature use? Does SuperDuper have this as well?
  9. blodwyn macrumors 65816

    Jul 28, 2004
    Portland, Oregon
    Chronosync will use as much disk space as the total of the files you're sync'ing, plus the archived versions of files you've deleted and/or updated (if you have the archive feature enabled).

    So if your external master drive containing your pictures is using 250GB, then the first time you sync to your new external backup (assuming you sync all the files and folders), the backup will also use up 250GB. As you grow your master, then the backup will track it every time you sync. Every file you delete or update will be copied to an Archive folder on both drives, so that will add to the disk space, although if you don't mess with the files much then this will be minimal.

    The difference between Chronosync and SuperDuper, is that SuperDuper is primarily an imaging program, so it will maintain an image of a disk, or a folder structure, and can do this to a schedule. I'm not aware that it can archive changes and deletions to files, it just maintains a faithful snapshot of the current state of the source. Also the selection options to build the selection of which files and folders to back up, and the filtering and processing options, are much less sophisticated than Chronosync.

    Chronosync, on the other hand, majors on the way you can select and filter and process the files that are sync'ed. For this reason I use both on my work system - I use Chronosync on a daily basis to sync and archive new and changed/deleted files in my main documents folder and a few databases in my Application Support folder, and use SuperDuper to take a weekly snapshot of my user folder, and a monthly bootable snapshot of my system disk.

    Both tools are great, and have some overlap, but really have a different emphasis on their approach to backing up. Both have free trials so you can test them out. For what you're doing I think I would go for Chronosync, as you don't need a bootable backup of the drive you're concerned with, and I like the archive option as a safety net against losing any valuable pictures.

    Hope this helps
  10. mfsnyc thread starter macrumors newbie

    Jul 23, 2008
    Yes- very helpful, thank you for taking the time to explain, now awaiting my new hard drive. Thanks again!:)
  11. neillondon macrumors newbie

    Jul 30, 2008
    Palm Beach
    So is it possible to use Disk Utility

    In response to:
    "Why not just use Disk Tools and set up the two external drives as a Raid 1 mirror or each other? It will only take a moment and seems like it would do what you want."

    Most of the SuperDuper and ChronoSync comments seem to be about copying from the HD to an external drive. I am trying to figure out if I can just use the Disk Utility to set up a second back up external drive to my primary backup external drive. Can I use Disk Utility in Mac OSX to just set up the two external drives to mirror each other? My issue is that I manually copy the new pix and files from my Mac HD to my primary external drive but I'd like that primary external drive to be an identical back up to the primary back up external drive.

    Do I really need the ChronSync or SuperDuper to set this up.

    MacBook Pro 10.4.11 2.16 Gz 2GB SDRAM

  12. alphaod macrumors Core


    Feb 9, 2008
    This is a n00b question myself here:

    Why can't you use the Disk Utility RAID feature?

    And what is 'Concatenated Disk Set'; know what concatenation is in programming so I'm assuming you're string multiple volumes with difference capacities?
  13. blodwyn macrumors 65816

    Jul 28, 2004
    Portland, Oregon
    If your objective is simply to set up a mirror on two external drives, then you can easily do this with disk utility.

    Advantages of mirroring:
    - Easy and free to set up
    - No user involvement in keeping the data mirror'ed
    - Protects against a single drive failure

    Disadvantages of mirroring:
    - Using 2 external USB drives can be troublesome, especially on a hub. If one drive spontaneously dismounts or doesn't wake from sleep (used to happen to me sometimes when I plugged another USB device in), then the mirror fails and you have to re-silver the mirror - i.e. you can't just reconnect the drive and it carries on. I had this happen too many times to make it fun. Rebuilding a 250GB mirror on external USB drives can take 18 hours. Firewire seems to be much more stable and faster
    - Mirroring does not protect against a file system corruption, you just get a mirror of the corruption
    - Mirroring does not protect against accidental file deletions or overwrites

    Using Chronosync or SuperDuper to sync, rather than mirror, negates all the problems above, but at the expense of having to set up a schedule to do it. Neither is limited to copying from the internal drive, you can use both utilities to sync an external to another external.

    So both mirroring and sync'ing have their merits and drawbacks. You just need to take an informed decision when you decide which way is best for you.
  14. blodwyn macrumors 65816

    Jul 28, 2004
    Portland, Oregon
    You can use the Disk Utility RAID feature, see above post.

    Your assumption about concatenated disk set is correct. Using Disk Utility to set up a RAID0 concatenated disk set, essentially adds the disks together to make one volume with the size of the individual disks added together, i.e. building a RAID0 with 2x 500GB drives will result in a 1TB volume (with usual disclaimer about different ways of calculating disk sizes).

    This sounds fabulous, and it is. The one drawback is that if either of the disks in the RAID0 concatenated volume fail, you lose everything on both disks. So your data is only safe while both disks stay functional. Either one fails - bye bye data.
  15. philradin macrumors newbie

    Aug 4, 2008
    Simialr Question

    Hi, I was intrigued by this question because I have a similar situation, but with a little twist.
    I would like to know if there is a piece of software that can examine and compare the contents of two external HD's and spit out an exception report of the differences between the contents of the two without having to manually campare the two drives folder by folder, document by document.
    Any ideas?
  16. blodwyn macrumors 65816

    Jul 28, 2004
    Portland, Oregon
    ChronoSync can maybe do what you want. If you set the source and destination to the root of each drive and do a trial sync, it will list the files that are not currently in sync with each other, and which is the latest version. There are probably other utilities that can do this.

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