Simple question about establishing a RAID mirror drive ;)

I've had my Mac Mini and Mac Book Pro for about a year now. I love them both! My question is about the 3 external hard drives I have attached to the Mac Mini and how to convert the largest from a Time Machine backup system to a RAID 1 (mirror image).

My hardware relevant to this question:
Mac Mini with a 120 GB internal HD that is currently at 37.92 GB used
WD My Book 1TB external HD that is currently at 657 GB used
Seagate 500 GB external HD that is currently at 6 GB used
Seagate 1.5 TB external HD used for Time Machine currently at 1.4 TB used

Due to the age of the Western Digital 1 TB drive that stores mostly video files I do not want to lose I'm becoming very concerned that it might be getting a little close to reaching the MTBF curve. I'm also afraid that I won't be able to figure out how to retrieve the data from the Time Machine backups if I have to replace the WD drive. I would like to convert the Seagate 1.5 TB drive (it's about 3 years newer than the WD) to a RAID level 1 so that I have a mirror image of not only the WD 1TB drive but also the internal and the Seagate 500 GB drives. I could just purchase the WD My Book 2 TB for Mac that I found at Best Buy today and probably muck my way through the issue with both a Time Machine and a RAID backup. I would prefer to use the existing hardware (more than adequate) to perform the task. I really want to make sure my data is safe and available in 2 places in case of a drive failure at any level so I can replace the bad drive and not lose anything important again. I lost a WD once and have never been able to replace the data that was stored on the drive. I want to make sure I do this correctly!

Please offer any and all detailed advice on how to make this conversion correctly so that I won't miss an important detail and then lose all the data on a drive.

Thanks for the help in advance!

I would go ahead and get the WD MyBook 2TB and something like Lacie's SilverKeeper (free, simple...no need for complexity here) to backup your 1TB and 500GB drives. You could do this on a weekly or daily basis. Not only will you have a backup but you'll have some level of versioning as well. If, say, you delete or lose a file on the 1TB drive you could go back to last week's backup and retrieve it.

I'm not sure you set up a multiple RAID1 arrays against the same drives like it sounds like you've suggested (the 1TB mirror and then the 500GB mirror) with OS Xs software RAID (...but TBH, I've not tried it).