You seem to be speaking about harddrives that have physically/electrically failed. Is this correct?
If that is the case, then there is probably little you can do after the fact. IF they are going to repair, you should find some contractual means to control (non-disclosure clause). You are really putting it all in thier hands.
For access failure, try BartPE or similar.
Bart is probably your best chance to get to data on any dead drive.
Looks like you also refered to destroying data before sending disk to repair. That would be the best solution, assuming that you don’t care about recovering the data. Degaussing would be good; there are lots of guides that will tell you about procedures and variables for different disks and data. Other than that, for non-functional disks, the other extreme would be a hammer. But since you are sending for repair, I’d guess that wasn’t an option.
For functional disks, there are utilities that will wipe the disks. You could do much of the same by filling disk with all “1”s then all “0”s repeatedly (7-10 times) to get same effect.
But the real oportunity would be proactive. Impement encryption of data on disks. There are numerous was to implement in DAS, NAS and SANs. You could even get some benefit by implementing a RAID, which splits data over multiple disks. However, RAID was designed to protect data, not secure it; check out the different RAID modes.
I’ve kind of bounced around on different topics related to your question. I hope some got close to your problem.