You often see data recovery companies claiming that they are approved data recovery partners to specific hard drive manufacturers. In all honesty most hard drive manufacturers will retain the warranty on hard drives that have been worked on by a reputable data recovery company, as long as a written report with a company letterhead is supplied on completion of the work.
Seagate on the other hand use their own Partner Recovery Process with registered data recovery companies to retain the warranty on the hard drive. These companies are usually more costly than the competition and it does not allow you the freedom of choice. With the low cost of replacement hard drives, it could work out cheaper using a data recovery company outside of the Seagate Recovery Process.
We have heard from customer feedback that it is the computer warranty that causes most issues. This is usually the case when you have bought a PC or Laptop from a seller and the hardware warranty also covers the internal hard drive. Sometimes to remove the problem hard drive, the PC or laptop has to be dismantled to gain access, and it is this process that can supposedly void your hardware warranty. I think the key is communication with the seller. As long as you explain the reason behind the removal of the hard drive and adhere to their process, then there should not be a problem.