I recently started using Intel Matrix Storage RAID solution that allowed me to use my 5 1TB drives for two RAID volumes. First one a 1TB RAID 0 striped across all 5 drives and second one a RAID 5 across the rest of the free space on all drives (around 2.85TB usable space).<p>The RAID 0 I use for OS, applications and games while the RAID 5 I use as a more-permanent type storage (photos, projects, etc). Now I do realize that running the OS and applications on RAID 0 across 5 drives is very dangerous, which is what brings up the following question.<p>Is there a reliable freeware realtime backup application that can backup a set of folders from one drive to another drive (no online backups needed)? I've already tried a few (Mozy, Yadis, Comodo Backup, GFI Backup, Idoo, Crash Plan) but none meet my requirements:<p>- Low CPU and RAM usage.<p>- Realtime Backups: as soon as a file is modified in the source folder, it is added to the backup queue which will be processed with the lowest priority when the CPU is idle. This backup queue should persist in cases of computer restarts and crashes (ie: the source and destination folders should always have the same set of files, except for the ones waiting in the backup queue).<p>- Incremental Backups: if only 10 bytes changed in a 1GB file, the app should only copy those 10 new bytes.<p>- Ability to back up locked and opened files (some apps, like Yadis, can't back up critical files like browser favorites).<p>- Ability to run as a service (no need for any user to log-in to have the app started).<p>Optional requirements:<p>- Compression of the destination into a well-known format (RAR, Zip) that can be directly read without the use of the application.<p>- Preset source folders (such as Browser Favorites, Game Saves, Application Settings, etc).<p>The idea is to use RAID 0 array as "semi-persistent RAM-like" storage which in case of a failure can be quickly rebuilt by reinstalling the OS, apps, games and copying over the settings, saves, favorites from RAID 5. I'm also thinking of taking this RAID 0 as RAM idea to the extreme with SSDs (as soon as we get some nice 6Gb/s SATA III SSDs out there), where a couple of SSDs chained in RAID 0 will work as yet another semi-persistent cache layer sitting between the RAM and the HD and serving all file requests for the OS, games, apps, etc.<p>Ideally, I need an application (or a loopback driver) that would convert any disk volume into a write-back, read-through cache backed by another disk volume as permanent store. If such an application doesn't exist, I'm thinking of writing one.