When trying to access the content of a hard-drive, you have to use the interface provided by said hard-drive. It usually comes with a firmware.
Currently, it is stated that this firmware will not allow data to be read without providing the correct password and that in case of 10 failed attempts, the data would be deleted.
To circumvent the restriction, you would have to :
- rewrite the firmware with a less restrictive one, but the current one will not allow being replaced unless the new one is correctly signed with Apple keys
- dismount the whole disks/ssd cells/whatever and put a new component to read from the storage. This requires very good knowledge on how the data has been written in the first place and eventually information that are store by the firmware in it's own memory (such as cell cycling for example). This operation can sometime be really difficult to perform, costly and might result in the destruction of the media.
Clearly the best way for any agency wanting to perform such hack is to ask for a new firmware, as it is easily replaceable and with no risk. Furthermore, the upgrade gets done by Apple themselves and not the agency.
The question about whether such agency are able to perform the latter solution remains open.