Question: I was looking for an external drive enclosure with a RAID controller and [found one in Newegg](http://www.newegg.com/Product/Product.aspx?Item=N82E16817145079).

Looking at Wikipedia, I cannot find information on this ["SAFE" terminology](http://en.wikipedia.org/wiki/SAFE).

Is this a replacement for RAID or something? If so, why?

The feature list for the enclosure state:

> 
> SAFE33 (1/3 to SAFE mode & 2/3 to BIG mode)
> 
> 
> SAFE50 (1/3 to SAFE mode & 1/2 to BIG mode)
> 
> 
> 

What does that even mean?

Answer: 
* `FAST` = RAID0
* `SAFE` = RAID1
* `BIG` = JBOD

`SAFE33` and `SAFE50` are modified and combined versions of RAID. You may want to refer to the [Sans Digital's RAID definitions](http://www.sansdigital.com/raid-diagrams.html) for more information. Official descriptions follow.

SAFE 33
RAID mode SAFE33 allows the creation of two hard drive volumes where 1/3 (33%) of the volume is used for mirroring and the rest is used for Spanning (Big). In comparison to SAFE (RAID 1) mode, which utilizes both hard drives for mirroring, SAFE33 increase storage space by setting aside a portion of disk space for regular data storage while performing mirror simultaneously.

![SAFE33 Diagram](https://i.stack.imgur.com/xmXAs.jpg)

SAFE 50
RAID mode SAFE50 allows the creation of two hard drive volumes where 1/2 (50%) of the volume is used for mirroring and the rest is used for Spanning (Big). In comparison to SAFE (RAID 1) mode, which utilizes both hard drives for mirroring, SAFE50 increase storage space by setting aside a portion of disk space for regular data storage while performing mirror simultaneously.

![SAFE50 Diagram](https://i.stack.imgur.com/hRfsc.jpg)
This is a stupid marketing term by the company in question. This thing has been possible on Unix-y systems for quite a long time with software RAID (DM on Linux, SVM on Solaris, etc.).

For example, take two disks (sda and sdb); create two partitions on each (sda1, sda2; sdb1, sdb2). Mirror the first partition on the drives:

$ mdadm --create /dev/md0 --level=mirror --raid-devices=2 /dev/sda1 /dev/sdb1

And then concatenate the the second partition on each drive:

$ sudo mdadm --create --verbose /dev/md1 --level=linear --raid-devices=2 /dev/sda2 /dev/sdb2

Ta da. An SVM example under Solaris is left as an exercise for the reader.

Just because the typical home user does not know how to do this under the most common operating system available does not make it novel, nor make it worth creating a new term for it IMHO.