Chris Lamb has suggested storing a GPG key on a RAID-5 device . The idea is that it can be stored on several physical block devices such that losing just one will not give the key to an attacker.
A default GPG secret key will be about 1.2K in size (3 sectors of a hard drive). A minimal key (with 1024 bit DSA keypair) will be 0.9K (2 sectors). I expect that very few people have secret keys greater than 4K in size.
To create a software RAID-5 device under Linux the tool mdadm is used. The default chunk size is 64K, so a 1.2K file will probably be on a single device. If you use the -c option of mdadm to specify a smaller chunk size then the smallest that is accepted is 4K which still permits a default GPG secret key to be on a single device. The Ext2 and Ext3 filesystems will always align such file data to a 4K boundary unless the device is smaller than a certain size (or a special mkfs option is used) to give a 1K block size for the filesystem. If an Ext2 or Ext3 filesystem is used with 1K blocks then you might get a 1.2K file split across multiple 4K RAID chunks.
So storing a GPG key on RAID-5 won’t prevent an attacker who steals one part from getting all the most valuable data. It will make it more inconvenient for them (if you are lucky it will prevent them getting all the data) and it will also make it difficult for the owner of the GPG key to determine which of the devices actually contains the secret data (probably all of them will end up having copies if you edit the secret key).
Now if RAID-5 did allow chunk sizes that were smaller than the secret key or if you have Ext2/3 with 1K blocks and get lucky with file fragmentation then the problem still isn’t solved. The reason is that you don’t require N-1 of the N disks to get some useful data out of a RAID-5 array (run strings on one element of a RAID-5 array to verify this). A single disk on it’s own will have some data that can be used, as file(1) can recognise GPG secret keys so you could just copy 1K chunks of data into separate files and use file to determine which (if any) has the data in question.
The really exciting question is, what do you get if you have the first 1K of a 1.2K GPG secret key? If it could be proved that the first 1K does not give an attacker any advantage then this might provide some benefit. But I think that this is a very dubious assumption, when dealing with such things it’s best to assume the worst. Assume that an attacker who has 1K out of 1.2K of secret data has the ability to reconstruct the rest. In that case the Linux kernel RAID-5 provides no benefit for storing a GPG secret key.
Just try not to get your devices that contain secret data stolen. Maybe a watch with a built-in USB device is a good idea. Thieves seem to be targetting mobile phones instead of watches nowadays and something that’s strapped to your wrist is difficult to lose.