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Looking for software to mirror two external hard drives.

level9tech asked
We have two USB hard drives that we need to mirror.  One hard drive is used to store RAW picture files within folders, and we would like to be able to plug in another hard drive into the same machine and a software find and update the changes so that each hard drive is an exact clone of the other.  We would need this to work on XP, Vista and 7.
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Gary CaseRetired
Most Valuable Expert 2013
Top Expert 2009

Microsoft's SyncToy, suggested above, or the free version of SyncBack [http://www.2brightsparks.com/freeware/freeware-hub.html ] will both do a good job of synchronizing the two disks.

Both run on XP, Vista, and '7.      I prefer SyncBack, as it has more options that let you "tweak" the synchronization settings -- but either will work fine.     You can run them "on demand" (i.e. manually), or you can simply set a scheduled task to automatically run the synchronization every day, every hour, or whatever you desire.
I like 2brightsparks SyncBack as well, user-friendly.  They call "proiles" are "jobs".  When it encounters copy errors it logs them and carries-on copying the rest.  One hiccup is the friendly html log file that is created and displayed will get overwritten the next time, so if you want to keep the log(s) you need to go to the program directory and copy/paste the cluster of files named after the profile(job).  The other hiccup is the modification date of target folders will be the date of copy not the date of the original same folder from source.

If the folder dates also matter to you, Microsoft's command-prompt RoboCopy will do it with the right command-line switches.  To "sync" you might need two passes, A to B and B to A.  Robocopy will make a logfile.  One thing about Robocopy you might want to adjust the number of retries on error or many many minutes can be wasted on a thousand retries on each file it has trouble copying.

Why might these programs have errors trying to copy some files?  Well, say if there were bad sectors affecting a file, or if the file was inaccessible, something like that.