Replaced optical drive with HDD, but alas 2nd HDD too slow!

You're hitting a known issue in this series system. The optical drive SATA interface is not able to support a SATA III (6.0 Gb/s) device. Auto setting drives (either HD or SSD) likewise won't work as the clocking of the port is not clean so it will not latch on SATA II (3.0 Gb/s) correctly. You will need to get a Fixed SATA II drive or see if the drive can be set to SATA II manually.

You should also note the optical drive port does not offer HD crash guard (only the primary HD port offers this). You should get a drive that has it onboard or be very careful as moving the system while it's running as it could damage the drive.

I don't recommend dual drive setups in this series and from our experience we prefer going with a SSHD like a Seagate laptop SSHD which give you the benifits of both a traditonal HD with its deep storage and the SSD cache to offer quicker boot and file access. On newer systems we are starting to put in large SSD's (1 & 2 TB drives) instead as the price has dropped enough to make worth doing.