Need to boot MacBook Pro into USB recovery mode I am trying to help someone recover data from a non-functional MBP (it was dropped).

I have the internal drive mounted in a sled and connected to another MBP via USB. I can boot off of the drive holding the Option key and get the login screen but, as the password is unknown, I need to boot the drive into the external drive's recovery partition, open up the Terminal and execute the resetpassword command.

How can I do this?
You don't need to boot from it to recover data - in fact it's probably far easier if you don't. 

If you have the drive mounted from an external USB, you can just Get Info &amp; check 'ignore ownership' - that will work unless the drive is encrypted. 

If it is, you need the password.