Mirrored RAID:  MediaKit reports block size error

I am trying to create a 2nd set up backup drives for my photos.  I have two new iomega 2TB drives, which look essentially identical to drives I'm currently using as my primary backups as a mirrored RAID set.
I can start the process with freshly erased and reformatted drives (with the default mac format, extended, journaled, unencrypted, not case-sensitive).  And after a minute or three, I see
"MediaKit reports block size error, usually caused by not being a multiple of 512."
The RAID options are Mirrored RAID, Mac extended journaled, and options settings are default.
I see several series of posts with complaints about encrypting RAIDs and disk block sizes, but not unencrypted errors.   I actually started out trying to do this with the 2006 MBP running 10.6.8 and got a different error:  "POSIX reports:  the operation couldn't be completed. Operation not permitted."  I wasn't sure whether the 2TB RAID I already have was set up iwth the older or newer computer--it was definitely before I put Lion on this one--so I tried this one and now have a different error.
Any idea what the problem might be? 

Update:  I spent some time on the phone with an Apple support RAID expert, and couldn't figure out what the error was; we couldn't bypass it by playing with partitions on the drives, or any of another couple of manuevers that I've already forgotten.  He noted that his own searches were showing a lot of mentions of similar problems but only with Iomega drives, and he was finding the same links I found earlier about problems creating encrypted drives.  Now trying to decide if it's worth throwing more good money after bad for a call with Iomega support, and waiting to see if the iomega forum is at all helpful.

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