They use a cheap home PC hardrive which gets
hot and needs afan to keep cool/this can be heard usually running.After a time the vents can get clogged with dust / or the fan can break.This can cause the box to overheat and damage harddrive memory.
  You can get new fans which are the same as PC fans
for £5 (try maplin or ebay)
 If the memory has gone you can get one from a old PC
for next to nothing, but somtimes you cant increase the memory to much as the latest drives run too fast.
 For a superior repair use a laptop hard drive then you can remove the fan and the system uses less electricity/runs silent/does not get hot
 the only other part needed to fit laptop hard drive is
a IDE converter £4 this is a computer ribbon which
converts the plug from standard memory to accept
laptop type 
(also applies to sky + plus boxes)
                
 Rob S
                
 July 2009