A general rule-of-thumb is to have matching disks in the array. The characteristics of each manufacture will differ, so the more similar the disks, the less like the RAID controller will have to deal with data corruption. That said, you don't have to use matched disks. For instance, make sure you replace a failed disk with another the same size or larger.
I wouldn't be concerned with 2 drives from the same manufacturer dying at the same time. The odds of that happening are tremendous -- probably equal to that of 2 drives from different manufacturers dying at the same time.
Now, if you're worried about protecting your data, please make sure that you don't solely rely on your RAID1 array. It is not a backup solution! Make sure you backup your important data, take it offline, and take it offsite.
I agree with the original answer, drives are pretty reliable now. But those IBM Deskstars were a well documented disaster, there's a reason Hitatchi bought IBM's drive unit. At least they readily replaced them under the warranty. – Auxonic – 2009-08-06T12:43:28.057
Thanks for the response. So hard drive quality has reached the point where I shouldn't be concerned about manufacturing/firmware defects affecting a "batch" of drives? I do plan to back up to an external USB drive and store it offsite. – Jamie Ide – 2009-07-16T14:17:37.450
I don't think the hard drive manufacturing process is perfect, so there are always going to be cases of "defects" in drives. Sure, these defects can often affect a batch of drives from one manufacturer, but the chances of those defects causing a failure at the same exact time are not likely. It's important to monitor the status of your array for this reason. If you discover a defective drive, get it replaced immediately! – Russ Warren – 2009-07-16T14:24:41.577
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I'd recommend reading this: http://www.dansdata.com/gz075.htm about drive reliability. Basically, the harder you use a drive the more likely it is to fail, but you shouldn't worry about it too much. It does happen sometimes that people have problems like with IBM Deskstars (Deathstars) that I had 4 of that worked happily for over 5 years, but people said were unreliable.
– Joshua Nurczyk – 2009-07-16T14:28:54.567