is there an app that will compare 2 drives and duplicate differences

What i mean to say is, an app that will allow you to pick 2 drives and tell it to compare them and any files that drive 2 does not have that drive 1 does, add them to drive 2. If thats not clear let m explain. I have to very large drives for storage with many gigs of files. They are meant to be storage for important files and the seconds of the 2 drives is there only for backup in case drive 1 dies. So it needs to have all the same files as drive one. But i'm constantly adding new files to drive one and unless i also add them to drive 2 i will lose files if 1 dies. Of course the simple solution would be every time i add files to 1, add them to 2 as well. Problem is, i often forget to do that so 2 is always missing some files that i have saved on 1. So i want an app that i can use every so often to evaluate both drives and allow me the option to have it copy any files it sees on 1 but not on 2 to 2. It would evaluate them by name and files size and if both match exactly it would copy. I there a app that will do this exactly? I know i could copy all files, but trust me i've tried and the computer can't handle doing that and usually freezes or something always happens to screw things up, not to mention it takes forever.

Well, you can try to do the syncing with Sync Toy 2.1. It is free, it is from Microsoft and it can be downloaded from Download: SyncToy 2.1 - Microsoft Download Center - Download Details.

Trust this was helpful.

Seemed like the perfect app, thanks. But i say seemed because used the echo function which seemed to be what i wanted, and when it was done it said done with errors and showed that it didn't copy any files. Unfortunately thats all it told me so i have no idea what the errors are. Any idea?

You could consider creating a RAID mirror then anything you add or delete to/from drive 1 will automatically be applied to drive 2. In fact, drive 2 won't be visible at all, the computer will see drive 2 only as a mirror of drive 1 :)

I have 3 RAID mirrors on my system - 6 drives that look like 3. In fact I have had 2 drives fail over time. When this happened the system just informed me of the failure, I bought a new drive, replaced the dead drive and the system rebuilt the new drive, all automatically. :)

Now THAT's security AND convenience....

I really don't want to go to any trouble. I just want the same files on the 2 drives.

Now i just ran it again and it did nothing, no changes. And this time no error. So i guess the first time it didn't work due to some error then decided to consider them synced even tho they are not. So i ran it a 3rd time only this time I created a new file on drive one and sure enough it copied it to drive 2. So does this mean that the app needs to initially be run with the 2 folders already 100% identical and then will only work after that?

EDIT: ok, now i'm more puzzled. I started looking at the 2 drives and it appears it DID work because i have a folder that i'm positive i've added files to without adding them to the same folder on drive 2. Yet the properties show the same amount of files and same size. So that folder is apparently synced which i assume means the rest of the filles and folders are also synced. But here are the 2 things that have me puzzled....1- when i look in my computer and compared the 2 drives, one shows 87 GB free and the other shows 76 GB frees. Whats even odder is it's the #2 drive which is the one i alway forget to add files to that says 76 GB free, meaning it's more full than drive 1 ! And remember, drive 2 is just a copy of 1 and it should have less files because i forget to add to it when i add to #1. Makes no sense ! 2-the folders appear to automatically sync now...if i delete a file from one, that delete happens to the other. Thats great and what i want.....but how does this happen without the synctoys app even running? is it that you only have to run it once after you select the folders and after that they are automatically synced and therefore the app is no longer needed unless you want to use it to sync other folders?

I did click on it but all it did was show what you have posted there but with zero everything....nothing showed successful or failed or anything. Just all zeros.
What do you mean by "255+ character path limit" ?

I see. Well, there are no subfolders i have deep enough to come anywhere near that many characters.
In any case, they are synced now and i'm good aside from not knowing why the 2 drives show much different total file amounts, one 76 GB free and the other about 80.

Oh geez....it never ends ! Now it is no longer syncing when i delete or add a file to either drive. As i said before, even with the app not running i could do anything to either drive and it would automatically do the same to the other. Now it's no longer doing that. Is this app just buggy or what?