UnRAID uses a drive or drives (which must be as large or larger than the biggest drive in the array) dedicated to storing parity information for the array. Parity is essentially using math to calculate and store a summary of data for each data sector onto a sector of a parity drive. If a drive fails, the controller can use what is essentially an algebraic equation to calculate the missing data -- or parity if that is what is missing -- to use to replace the data (slowly) until the failed drive is replaced and rebuilt. You can imagine it as a gigantic, but simple sudoku puzzle.

UnRaid essentially creates an array of drives with the same directory structure and will put each new file onto the drive that has the most space. It will then present the array as a single structure showing the files that appear on all drives in any given folder. A critical advantage to this is that even if multiple drives fail at once, you still have access to all the data on the working drives. And you can access it from any PC by simply attaching the working drives.

A minor consideration in most cases is that UnRaid will only spin up the drive where the data is located to share it. Particularly in the case of a media storage system, this can greatly reduce the number of hours the drives spin on average. An array with 4 data drives and one parity drive will only age the data drives 1 hour for every 4 that the array is being used.

Drobo data storage units use a different and proprietary system where each file is written to 2 different drives and an inventory of where each is is saved in a table rather than a sector-based parity system. In a 5 drive array, a first file might be saved to drives 2 and 5, and then a second file might be saved to 3 and 5. Should drive 5 fail, the first file will be read from drive 2 and copied to whichever of drives 1,3, & 4 have the most space, and then the second file will similarly be copied from 3 to whichever of 1,2, & 4. Even though files are completely saved on drives, due to the proprietary format Drobo uses, you cannot read the data from the drives on your PC. Professional data recovery services can perform this service however.