Using wd external drives in raid

I recently purchased 2 WD Elements 2.5 TB drives, and I am running OSX Lion. I have repeated tried to create a raid, and I get this error:

POSIX reports: The operation couldn't be completed. Operation not permitted.

I am using DiskUtility. I have tried mirrored, striped, and concatenated raid types. I have also tried extended journaled, extended case sensitive journaled, and FAT format types.

I've read some post that suggest you cannot use WD external drives for a raid? I am looking for help in getting this working. I prefer a mirrored raid for backups in case one drive goes out. I am using this for storing family photos, ect.
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More about using external drives raid
  1. First, I could be wrong, but i have never heard of any OS supporting USB based disk RAID.

    Second, for the best security, external USB drives should ONLY be connected while backing up, and should be disconnected at other times. This helps eliminate the possibility of accidental erasure of files and from file corruption from a crashing program or virus.
  2. Does OSX Lion even support HDD's above 2TB? Also, external HDD's don't even approach the read/write speeds of internal hard drives
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