What part type is denoted by SKx?
while taking apart a small external hard drive I found a component which looks to be made of ceramic labelled SK1


It's a shock sensor used by the hard drive controller (hence the choice of "SK" for the designator makes sense  - ShocK).

For example, here is the product page for this type of ceramic-package shock / vibration sensor, made by Murata, containing cantilever-mounted piezo elements:

https://www.murata.com/en-eu/products/sensor/shock/consumer

Quoting from that web page:


  
    Murata shock sensors find use in many computer hard disk drives. They detect impacts - for example, when the storage falls to the floor - in a fraction of a second and prevent read and write errors.