It's possible the external hard drives have been compromised in the company I work for. It would be a huge waste of money to replace them all without detection and if the current ones are compromised, the new hard drives could be compromised again.
The hard drives have already been scanned with Anti Virus / Anti Malware software. Those softwares would not detect firmware / hardware hacks.
Could you suggest how to detect if the hardware / firmware of the hard drives have been compromised?