Pushing past 3TB of disk space

I have a Mac Pro connected to a drive enclosure containing two 3TB disks. The disks were originally set to a RAID 1 configuration using Disk Utility, so that each one backs up the other automatically. I am now fast approaching the point where my storage needs exceed the 3TB. The enclosure has room for another two disks and I’m thinking about moving up to a RAID 5 configuration. But Disk Utility doesn’t seem to support this. Is there a thirdparty utility that will do this for me? Gareth Woodward Yes. SoftRAID (£126, softraid.com) will do this but I’m not sure it’s really what you want. RAID 5 distributes your data evenly across all the disks in the array, with some extra parity data included so that the array can continue to function if any single drive fails. If you added another two 3TB drives, the parity information would take up the equivalent of one disk’s worth, leaving you with 9TB of storage – which seems like a good deal, but RAID 5 isn’t a backup. No flavour of RAID protects your data. What they do is protect access to your data. If a drive fails, you can still get at all your data with no interruption. But until you have replaced that drive and waited the day or two for the parity information to be rebuilt, your drives are effectively in a RAID 0 array. Suffer a second drive failure in this period and you’d lose all your data, because every file is distributed across the three remaining disks. With one of those missing, you have just two-thirds of every file. Which is the same as no file at all.

A safer option is to set up two of the disks as a single 6TB volume (a ‘concatenated disk set’ in Disk Utility) to store files. Set up the remaining disks in the same way, and use a utility such as Carbon Copy Cloner (£27.50, bombich.com) to periodically back up the first volume to the second one. This gives you an actual backup that will protect your 6TB of data against the loss of up to two disks in the same set, though still not against one disk in each array.