Backing up to something other than a harddrive has the disadvantage that you can't backup everything up-to-the-minute. Backing up using RAID 1 or another form of mirroring has the disadvantage that it reduces the amount of logical harddrive space to a fraction of its physical combined size. You could get the best of both worlds by: a) backing up new data to DVD-R or maybe DVD-RW periodically, for example once a day b) mirroring what *isn't* yet backed up on other harddrive(s) The filesystem would keep track of everything that's backed up and automatically mirror files that are added or changed, until they're backed up. The mirrored data would exist as separate files, or one large file, on the (partially) mirrored harddrive(s) so that it can grow and shrink and not need the entire harddrive or even a fixed-sized partition on it. I'm not sure how incremental disc backups are usually handled, so that you don't have to go back 10 years and put in 100 different discs in series to reconstruct the original data, but I'm sure this question has already been handled. But if not, I have some ideas. 1) when possible, erase the entire disc and rewrite it. Make sure you don't rewrite a disc too many times so that it doesn't work. Have the software keep track of how many times it's been rewritten. Also use multisession for incremental changes. If used in combination with erasing, a multisession disc wouldn't have to be erased until it gets full due to incremental additions. When you can't write incremental changes to an existing disc, write them to a new one. 2) Have a limit to how many incremental backup discs a given source tree can depend on compared to the actual number of discs needed to store the entire tree. The software should know exactly what's on what disc so it can automatically enforce this limit. When it's too many, it can start over from the beginning and backup to new discs, or better, erase the old discs and write over them. OR it can only rewrite just enough to keep the number of backup discs below the limit. Don't make an entire backup session fail if one disc fails to backup. Keep the data about the files already backup in the backup software's database, and have the option to skip the offending files and/or try a new disc. On every failed file dialog, give the ption to automatically skip all files that fail for that disc and try them later on another disc, an option to just skip them permanently, and an option to zip the file or zip all failed files for that backup and try to backup the zip file, because the problem may have been a filename/path incompatibility that we just didn't catch. Oh, whenever the backup program detects that a filename/path isn't compatible with the disc's filesystem, it zips it in a compatible filename. The two constraints that I know of are which characters are allowed and how long the full path can be. If the user discovers that one of the discs in a backup is bad, have the option to write the same data the hard drive to a new disc from the hard drive. The discs should be labeled with numbers so the user knows which disc is which in the database. Those are used for restoring files, too. Store the backup database and optionally the backup software too on one of the discs. ability to change how much data we put on the current disc. someone in a forum said that 25GB m-discs only worked for him if he stored 22.5GB on them or less. i think he said that the package actually said the discs stored x amount which was somewhat less than 25GB. can exclude specific files from being backed up in any directory designated for backup Option to backup all files in zip files, or alternatively only backup files that have filenames/paths that aren't compatible with the disc's filesystem stored in zip files. The backup database remembers which files are in zips and which are plain. Can break up large files across discs. Do this automatically for files too big for a single disc, and have an option of whether to do it for smaller files to save space. Don't save split files in their original filenames, of course, have a special file extension for split files, with numbers appended for which file it is in the series. When making a zip file or a split file, do a check to make sure the filename doesn't clash with some other filename already on the disc, and if it does, store it under a different filename. Can select which files are backip up on the current disc in a way to use maximum space. Don't require all backup discs to be the same size. Must have some way of handling files being backed up changing as we're doing our backup. Files changing that we haven't gotten to yet don't matter, but files changing as we're backing up the current disc doo. I guess we could just copy all the files for the current disc to a temp directory. But then, for Blu-Rays, that can take up to 100GB. The user would have to spend a lot of time waiting for files to copy. Another option is to dynamically reorganize the backup (should only have to reorganize what's not backup yet on the current disc) when it's detected that files we're currently backing up have changed. That's actually the better option. Of course, put a write lock on a file whenever we're currently reading it for backup, and make sure we don't do this *after* we catalogue the file in our database.` Support CD, DVD, and Blu-Ray. And M-discs if that needs special support (I don't think it does). When backing up to anything but an optical drive (actually, to anything that allows us to delete files), have options to keep in storage X versions of files that are older than Y amount of time, and be able to define these values for multiple tiers of age. E.g., all versions newer than 10 days old, 3 versions newer than a year old but older than 10 days, etc. Duduplicate files across the whole backup. Or optionally deduplicate actual file data across the whole backup using N-sized blocks of data, where N is configurable. This would require storing all files in a custom format, or at least all files with deduplication in them. Have different options for the disc's filesystem (a program setting that can be changed between discs if the user needs to), especially options that allow more filename/path compatibility with the source files.