Anatomy of a Storage Drive: Hard Disk Drives

Squuiid

TS Enthusiast
“ This 3 TB drive has three platters, so each one must store 500 MB on each side.”

That doesn’t quite add up ;)
 

neeyik

TS Evangelist
Staff member
Oops! Lemme just slide that M back up the alphabet a bit...

Edit: All fixed now; thanks Squuiid (y) (Y)
 

unseen

TS Rookie
I may be wrong, but I think platters on modern HDD's are made out of glass, because glass can have an extremely even surface, and the physical expansion under different temperatures is minimal. The old ones are made out of metal.
 

Nunya

TS Rookie
There's something I've wondered about for years. I once took apart a dead HDD to get at the magnets inside. What I found were two extremely powerful neodymium magnets similar to the ones in your pictures. And when I say powerful, these magnets were so strong that when they, unfortunately, came together, I was never again able to separate them.

My question is this: How could two magnets that powerful live inside an HDD and not create a magnetic field inside the case that would erase the platters?

Thank you.
 

neeyik

TS Evangelist
Staff member
I may be wrong, but I think platters on modern HDD's are made out of glass, because glass can have an extremely even surface, and the physical expansion under different temperatures is minimal. The old ones are made out of metal.
There's certain an overall industry move towards the wholesale use of glass, but aluminium is still cheaper (albeit not by much), so for the small and ultra cheap markets, metal platters are still used.

My question is this: How could two magnets that powerful live inside an HDD and not create a magnetic field inside the case that would erase the platters?
The clue is in the fact that the read/write heads in a hard drive have to be within a handful of nanometres above the platter surface to affect the direction of the magnetic domains in the 'data' material. On that scale, the permanent magnets are effective on the other side of the galaxy, so despite their field strength, they're nowhere near close enough to affect the data. Also, the field between the magnets is very constrained - it's strong directly between them, but the field scales down rapidly outside of this region.