I know this sounds random, but I had a hard drive that died awhile back, so I took it apart for fun to see what was inside. In hindsight, that might not have been a good idea, because who know what kinds of nasty chemicals and stuff could have been in there. Anyway, I kept the platters because I thought they looked cool, and I was thinking about using them for coasters. But can anyone here think of a health reason not to do so? According to wikipedia, the platters are made of glass or aluminum disks, coated in 10-20 nm of cobalt, with a layer of carbon for protection. Each of those things sound ok to me, but I thought I would check here anyway.
Even if there is something bad on them, just don't lick the bottom of your glass. There are some very powerful magnets in there to, but I haven't heard of anyone getting sick from taking a drive apart. You should be ok, just don't lick or eat the parts.
The platters are actually quite strong and are unlikely to shatter, I have not seen any glass ones yet. When I get a faulty hard drive I take out the magnets, they are incredibly strong and can provide hours of amusement.
I don’t know what material the magnets are made of but I expect that it is an alloy of some rare earth metals only mined in China. I highly doubt that they are at all radioactive because the radiation could interfere with the reliability of the electronics. The biggest danger from the magnets is that of getting your fingers trapped between two of them and for that reason it is probably ill advised to give them to children to play with.