It will not affect the speed of the computer. Only one disk can be used at a time, so your mass storage data transfer will be the same with both drives. This might be different if you place two drives in a RAID 0 configuration, but according to the specs on the E520, only RAID 1 (Mirroring) is supported. Therefore, the speed will be the same. You will, however, have a lot more room to store data.
I installed my 1T second drive into SATA4. I enabled SATA4 in BIOS. I went into Disk Manager and set it to format. After 24 hours it was only 56% complete so I poked around the web and realized I should have partitioned it. So I cancelled the format and rebooted. I went back to Disk Managwer and set it to partition 300MB and clicked "Quick Format." I went to unload the dishwasher and when I returned the new disk no longer appeared in Disk Manager. I rebooted again but now BIOS doesn't recognize it. I had to disable SATA4 to get the CPU to boot up again.
What did I do wrong and what do I do now?
My first impression of your post is that the hard drive has failed. Formatting and partitioning should not affect the detection of the hard drive in the computer BIOS.
Before giving up on it however, you should try connecting the drive to SATA 1. When you use the hard drives as individual devices the Dell BIOS prefers hard drives to be connected to one of the first two ports. The ports seen as SATA 4 and SATA 5 are intended for your DVD/CD drives (the E520 does not have support for ports SATA 2 or SATA 3). Once you have the hard drive connected to SATA 1, reset the CMOS (NVRAM) by using either one of the keyboard methods found in Chris M's post here, or by removing the CMOS battery for about five minutes.
EDIT: Specifically, see the link marked Clearing NVRAM in Chris M's post.
The new HD is dead. BIOS did not recognize it in SATA1. I jumped the CMOS. I moved my CD/DVD to SATA4 and it is recognized, so there is nothing wrong with the SATA4 port. I'll send the HD back to Hitachi on Tuesday. I am in your debt.
Would it be possible to hook up a 3rd hard drive and put it on the DVD/CD cable instead? Would I need to remove the BATTERY to reset the BIOS as suggested? Thoughts? I have 2 drives at the moment.