I agree with bencurtis about accidentally switching off a set of drives when using internal and external. Will cause a bad crash of data.
However, Highpoint Technologies makes great RAID cards and they constantly upgrade the software to coincide with OSX releases. They have very good Mac support to boot
You probably will need the RocketRaid 2322 which has 4 ports SATA/SAS internal and 4 ports external.
I have a 6 raid drive set in my MacPro. I use the 2320 model which has all 8 ports internal.
In order to fit all 6 drives, I utilize the 4 internal bays PLUS the lower Optical drive bay. However, in order to get the internal bays to connect to the PCI card (normally, the hard drives connect directly onto the MacPro motherboard which means you can't attach an SATA cable to connect to the 2320 card) , I use the MaxConnect SATA link (only $89 for this). You can search on Ebay to get used pricing on the 2322 which pops up often. I bought mine for $180 which was a pull off another system. This card although over $400 new, rarely goes over $200 on Ebay.
Max Connect -
www.maxupgrades.com"
HighPoint -
www.highpoint-tech.com"