First,  make sure that the failed drives are covered by hotspares.  If the hotspare has finished building for the failed drive and the unused drives are of the same size as the failed one, you can replace them.  Just make sure that the drives that you are using are not part of a raid group or anything.   They should be listed as unbound on the disk summarry page.  If possible, i would use the NST to do it.
Need some help on replacing a drive. I just took over a site that has 2 older CX700 arrays that are not on support. Both have failed drives. The system has drives that we are not using. What would be the proceedure to remove 2 of the unused drives and use them to replace the bad drives?   Thanks,  Hank
Replacing CX Failed disk