So I am re-evaluting my backup process again currently, have just brought in a secondary RAID device to put various older HDDs. Now what I am looking for is some simple software to sync the first RAID device to the second - as part of a backup (I have external drives as true timepoint backups, but this step is in additon). I have tried a couple of free software years ago (the M$ free one SyncToy for instance) and they had problems with the drive letters not being fixed. So I am looking for peoples opinions on the software, free or otherwise, to do this kind of file syncing over devices, I can of course fix the drive letters for each device if need be. Currently the one that jumps up on most lists is: FreeFileSync
Just to confirm the criteria is sync'ing files rather than copy the whole as backup, and speed is less of a concern as to security of the copy.
What are your thoughts?

I'm using FreeFileSync. Often updated. Friendly interface, very transparent about what it copies or replaces, where, why, and how much data it means. Also, easily lets you select what to ignore.

My only gripe with it is that older versions used to make my anti-adware freak out a bit for some reasons, possibly with falso positives. But this seems to have calmed down for a while.

I'm also curious to know if there is something (free) better than FreeFileSync, but I wouldn't be much surprised if there wasn't.

I did a similar search recently myself and settled on Allway Sync. The free version has some restrictions on the total number of files you can sync in a 1 month period.but the pro version is still pretty inexpensive.

rsync ftw!

Thanks for your suggestions. I will have a try with FreeFileSync for a couple of backups and see how it goes.