Hello Mohamed,
1-Both disks are not compatible due to the form factor, one disk is 2.5" and the other 3.5"
ST9500620NS is PID "A03-D500GC3=" << 2.5" disk
ST3500514NS is PID "R200-D500GCSATA03" << 3.5" disk
See this:
http://www.cisco.com/en/US/prod/collateral/ps10265/ps10493/spec_sheet_c17-644227.pdf
Keep in mind "R200-D500GCSATA03" is end of sale already.
2-Disk are hot swappable, all of them. When there is faulty disk and you replace it, the rebuild process should begin automatically, however, udner some circumstances, the rebuild needs to be triggered manually.
Rate all helpful answers.
-Kenny