How to automatically mirror two identical external harddrives? (Raid?)

How many drives do you have? Are there two 2 TB drives or four 1 TB drives? Not clear as you stated, "I ran out of space and just got me another Western digital MyBook Studio Edition II (2 x 1TB harddisk) which gives me a total of 4TB." By my arithmetic that's a total of 2 TBs.

You can create a mirrored raid of two 2 TB drives that will provide 2 TBs of storage with one drive mirroring the other. The two drives should be the same size and preferably the same make and model. You will need an external enclosure for two drives or one that contains RAID hardware for a mirrored RAID. You can find an enclosure at OWC.

Creating a RAID requires starting with two blank drives that have been partitioned using GUID and formatted Mac OS Extended (Journaled.) When the RAID is created both drives will be erased.

RAID Basics

For basic definitions and discussion of what a RAID is and the different types of RAIDs see RAIDs. Additional discussions plus advantages and disadvantages of RAIDs and different RAID arrays see:

RAID Tutorial;
RAID Array and Server: Hardware and Service Comparison>.

Hardware or Software RAID?

RAID Hardware Vs RAID Software - What is your best option?

RAID is a method of combining multiple disk drives into a single entity in order to improve the overall performance and reliability of your system. The different options for combining the disks are referred to as RAID levels. There are several different levels of RAID available depending on the needs of your system. One of the options available to you is whether you should use a Hardware RAID solution or a Software RAID solution.

RAID Hardware is always a disk controller to which you can cable up the disk drives. RAID Software is a set of kernel modules coupled together with management utilities that implement RAID in Software and require no additional hardware.

Pros and cons

Software RAID is more flexible than Hardware RAID. Software RAID is also considerably less expensive. On the other hand, a Software RAID system requires more CPU cycles and power to run well than a comparable Hardware RAID System. Also, because Software RAID operates on a partition by partition basis where a number of individual disk partitions are grouped together as opposed to Hardware RAID systems which generally group together entire disk drives, Software RAID tends be slightly more complicated to run. This is because it has more available configurations and options. An added benefit to the slightly more expensive Hardware RAID solution is that many Hardware RAID systems incorporate features that are specialized for optimizing the performance of your system.

For more detailed information on the differences between Software RAID and Hardware RAID you may want to read: Hardware RAID vs. Software RAID: Which Implementation is Best for my Application?

OK. Got the picture. Now I don't know if Disk Utility will do this properly but you can try. Start with all four drives freshly partitioned and formatted. Create a striped RAID in each box of two drives. This creates two 2 GB arrays. Let's call them Array One and Array Two. Now create a mirrored RAID whose members will be Array One and Array Two. Essentially this is what some call a RAID 1+0. The result is a mirrored array with a 2 GB capacity. Now it's my understanding that the Disk Utility RAID software will support such a concoction, but I've never done one so I can't tell you that it works. If you have a problem with it then you may need to use third-party RAID software that does support such an array.

As for speed. As a rough approximation putting two drives into a striped RAID will result in a doubling of the throughput of a single drive. Mirroring two drives in a RAID does not result in any speed increase and may actually result in a slight decrease in throughput. So my expectation of the above concoction's throughput is that it will be faster than a single drive but not twice the throughput of a single drive. And, for any striped array the throughput declines as the array space is used up. Now, it's difficult to say what you will experience doing all this on FW800 which is essentially an interface that is slower than any single modern hard drive. It's the port's capability that will prove the limiting factor no matter how fast those drives are.

As for backup software I would recommend using a third-party backup utility in which you can set up a regular backup schedule that you can set and forget. Here are some suggestions:

Backup Software Recommendations

Backuplist
Carbon Copy Cloner
Data Backup
Deja Vu
iBackup
JaBack
Silver Keeper
MimMac
Retrospect
Super Flexible File Synchronizer
SuperDuper!
Synchronize Pro! X
SyncTwoFolders
Synk Pro
Synk Standard
Tri-Backup

Others may be found at VersionTracker or MacUpdate.

Visit The XLab FAQs and read the FAQ on backup and restore. Also read How to Back Up and Restore Your Files.

Message was edited by: Kappy

The MyBook drives have a utility that permit you to configure them to run as RAID0 or RAID1. The default is RAID0.

RAID0 means that data is spread across both drives, if either drive fails, all data is lost. The reason people use RAID0 is because it distributes the load of disk activity across two disks, increasing performance at the cost of decreased reliability.

RAID1 means that the drives mirror each other. One is always an exact duplicate of the other, and if one fails, the remaining drive will (hopefully) contain a usable copy of the data (which will be mirrored to a new drive when you replace the failed one). This increases reliability at the cost that you use up half your storage as a duplicate. Unlike other types of RAID, RAID1 doesn't include any internal integrity checks (parity checks) to further assure the data integrity. However, it's probably what you are thinking of. The WDC RAID Manager app that comes with the drives (or can be downloaded from wdc.com) is what you use to switch the drives to RAID1.

You will setup each MyBook to be either RAID1 or RAID0 independently, so you will ultimately have two volumes. Personally, I use a MyBook in RAID0 as my universal scratch space (particularly for video editing). I have a NAS that has RAID1 that I use for storage of less transient information.

RAID and backup serve different purposes. RAID boost performance and/or acts as a hedge against drive failure. Backup permits you to keep one or more older versions of data in the event you delete or modify the data (something RAID doesn't do -- though there are some filesystems that do it).