The following only applies to "spinning" hard drives (not SSDs, Flash Drives, SD/Micro SD cards, Cfast/CF Cards):
About 25% of drives we receive have bad heads (these are most often drives that have been dropped and/or are clicking). We always try to provide our customers with the option of an affordable "partial" recovery, by disabling the bad head(s) and recovering data using the remaining good heads.
However, sometimes the only way to recover any data is by replacing parts (i.e.; "heads") inside the drive. In these cases, we will offer you the option of paying for the necessary donor part after we find the best matching drive online. You can also decline our recovery attempt (for no charge, excluding up-front fees) if the cost of donor parts is too expensive.