I will edit this post with a picture of the oxide on the broken magnets, Yes it feels similar to magnesium, when they break they sparkle a bunch.
I haven't heard of terbium magnets either. I have none of the drives whole, as they were meant to be destroyed. I can however get a picture from google.
Would be nice if it was terbium, wasn't happy to destroy them given they so much better than ssd garbage.
https://www.allhdd.com/ibm-46m7030-hard-drive/
I could however post pictures of the covers, platters and pcb's.
Btw that bga chip i imagined was on older scsi not these, they still have a decent bunch of stuff.
Now that you say it, i might take a bit and see how it reacts with water.
Edit: In two minutes I spent along with it i haven't seen any bubbles but the ones that came when i dropped it in the water, i'll sit with it for a while.
Edit2: No reaction with the water, but took better pics.
Edit3: I first upload of the drive was from 2013 while mine where end of 2008 and 2009. This one looks more like it.
Edit4: One hour and no bubbling, ambient temp is around 25ºc i guess and the bit i tossed is small, i will get some hot water. Thanks for the advice.
After adding the hot water i'm thinking this is praseodymium, still not making sense for me, upon reacting with air becomes red like terbium, yet in the warm water is getting darker . I guess color left by end of the day will clarify this.