David,
As Steven recommened I went with the RAID 1E option on my R4. I've extreamly happy and thanks to Steven for all the great advise! I also have my OS and boot from the R4 with no problems at all. I think I've had it for around 3 months now and have never experienced a single disconnect of the drive (touch wood).
Unlike Steven I switch mine off every night and boot in the morning.. no problems at all!.. you just have to remember to boot the RAID first THEN the imac and visa versa when switching off.
Just as an FYI.. here's the procedure that I used to connect and install the R4 as my startup disk.. posting as someone somewhere may find it helpful :-)
1) Fire up imac.
2) Connect of R4 to the imac with thunderbolt cable and power up (use left hand thunderbolt port on the R4)
3) Copy over the Pegasus RAID utility from the R4 and save it on the internal drive and or a Disk for safe keeping
4) Format the RAID using the Pegasus RAID utility for 1E with sector and stripe 512 and wait to complete (10 hours)
5) When format is complete, check that the drive appears on disk utilities
6) Download the Osx Lion Installer from apps store
7) Connect a Time Machine backup disk to the imac
8) When it asks which disk to install on then choose the RAID 1E
9) When the install walk through screen prompt asks if you want to import data then choose time machine backup
10) Finish install process
11) Go to system Preferences and Startup Disk. Select R4 HD and restart. The machine should boot from the R4 and wont touch the internal drive
12) Carry out system check, everything opens and runs OK, closed down OK, no crashes etc
13) Complete all available updates of SW and re run systems check
14) Download Black Magic design speed test from the App Store and benchmark the read write speeds. Record results.
15) Connect up 2 x 2TB WD External HD's to USB ports and add them to time machine backup and Deja Vu backup respectivly. Make sure that the second Deja Vu backup is in the TM exclusion list (which I didn't, and again Steven helped me with! :-)
Steve.