I've got two S3 Tivos with Fantom 1TB external drives attached and they're working just fine. Both S3's have had their internal drives upgraded to 500GB and 750GB. I've been running dual drive Tivo setups for almost 10 years and I've only had one hard drive failure that caused me any data loss in all that time, and I believe the drive that failed was the OEM drive that came with the Tivo. I've literally owned several dozen Tivos of all varieties over that period. In other words, there's no real reason to be concerned about data loss or drive failures by adding a 2nd drive. You stand just as much chance of a drive failure with a single drive as you do with two.
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