Core Storage Encryption on RAID External Drive

If the RAID is a hardware RAID this should effectively be invisible to the Mac operating system and therefore not cause any issues when then encrypting it.

With regards to using Apple Software RAID, I believe that the way Apple now do this is different to how it used to be and that these days it would start off as a CoreStorage volume when it creates the RAID thereby also simplifying matters. I suspect your RAID was created using an older version of Disk Utility.

In terms of safeness of FileVault2 vs. TrueCrypt. TrueCrypt was discontinued and did not support encrypting the Mac boot drive. I would therefore not recommend using TrueCrypt. There are various other similar packages e.g. Sophos Safeguard, CheckPoint Full Disk Encryption, PGP Disk and so on. All of these however suffer from the fact that when Apple releases an update to their operating system they are initially not compatible and upgrading them can often require fully decrypting the drive, uninstall their software, installing a newer compatible version, re-encrypting the drive in other words lots and lots of pain.

FileVault2 uses the same main encryption algorithm as used by most of these other tools which is AES (Advanced Encryption Standard), Apple have had their FileVault2 implementation of this certified by various government bodies and the most well known one is the US one referred to as FIPS 140-2.

Note: Even though there may be no changes in Apple's implementation each time they issue a new version of OS X the way these certifications work requires that Apple's implementation has to be re-certified each time. (This would presumably apply to other products as well e.g. Windows BitLocker.)

So getting back to how safe is FileVault2. It is safe in that it uses the same underlying encryption i.e. AES as used by most other products, it is safe in that it will always be compatible with the Mac operating system, it is safe in that it has been and will be certified, it is safe in that it is used by government departments themselves. I would not be able to say myself if there are any backdoors in it but Apple certainly would not knowingly have created such a backdoor - just look at all the press coverage over Apple being sued by the FBI and Justice Department over encrypted iPhones. I would also not be able to say that government organisations like the NSA or GCHQ might or might not be able to crack the encryption even without a back door again this is going to be as true or not as for other products.

Nobody knows why Apple does anything. They rarely publish their reasoning for anything at all. Since none of us are Apple employees, we wouldn't have any idea why Apple made Disk Utility work the way it does. Any response you get would be speculation about Apple's policies which is completely forbidden on this forum.

That's just a plain old regular logical volume lol. No raid there. Also, RAID 0 is "striped", not "mirrored", which I'm guessing is the reason they did not enable FileVault support by default.