Dear BB,
I am so sorry if I made you paranoid about mixed boot environments with both OS X and OS 9. Some here have had years without issue when having dual boot systems with OS X and OS 9, that being said, if you search the forum, the dreaded btree 0,0 error does NOT occur in environments if OS X and OS 9 systems and external hard drives are never mixed:
http://macos9lives.com/smforum/index.php/topic,2830.msg19106.html#msg19106OS X via spotlight or simply mounting OS 9 volumes can write information to the header volume of the drive that OS 9 "first aid" will flag as an error and want to repair...however, most of the time the data can be copied off the volume, the drive can be reinitialized, and the data copied back (but a real pain in the ass). Now, for safety sake, if you wish to keep OS X away from OS 9, then these weird anomalies when using the OS 9 disk "first aid" will NOT occur. Also, it appears that very early versions of OS X, like 10.1,10.2, and 10.3 may not create as many issues as 10.4 or 10.5 (but this has to be verified). Lasty, long files name that look fine in OS X... will display weird characters in OS 9... that alone, is at the very least, annoying.
Now on to defrag, some like Knez, will actually boot to OS X via a DVD and run defragment utilities on large OS 9 volumes (300GB or more)... I have never been that bold.
Also, defraging will only be needed if you are using the ext HD volumes to record and edit, if you are simply copying finished projects to the external volumes, then you will NEVER have to defrag them... this is a subtle, but important point, since you will have to make any volumes 190GB or less if you plan on defragmenting them in the future under OS 9, since Norton Speeddisk will bomb with "out of memory" errors on larger volumes. I am guessing it cannot load all the blocks in memory that is have to relocate on volumes that are larger than 200 GB, so 190 is the safe bet.
On to OWC, I have never seen the "hardware" based RAID in their External FW enclosures NOT work with OS9. As IIO mentioned, you said "stripping" (RAID 0) in one of you posts, which is radically different that RAID 1 (mirroring). Stripping will give you 2 TB of high speed storage (if you use 2 one TB drives), but if either drive fails, you are fuckola... all data gone; if you RAID 2 one TB drives with RAID 1 (mirror) you get 1 TB total storage and you still have your stuff if either drive fails. Remember, the OWC techs are great, but I am sure there are some kids there that never even saw OS 9, so don't rely on what they say or even what they print since they do not have OS 9 in the mix... it is like buying some new PC hardware and seeing if the literature says it will work with DOS. You have to try it, in general, FW400 is a very safe bet for OS 9, if it works initially, it won't have some crazy issues later, it is doesn't you'll know right away.
As a last note, I have done this with the OWC Mini Dual metal (but have not tested the maximus). I have taken a single 1 TB Hard drive, partitioned it internally in a G4 and setup all my partitions under OS 9 and even copied data to a few volumes, then I have put it in a RAID case with an
additional blank hard drive and the RAID switch flipped to mirror) and viola !!!! It copied and mirrored all the stuff from the drive with data including partitions to the blank drive and made a healthy mirror set. So that simulated a drive failure (one drive with stuff and one new) and the case did all the work