I have an asus p5k-e mobo, and have always had a RAID 1 array on it of Seagate 320 GB disks. This has always worked fine, but I thought I should get another HDD for storage and backup. It's a Samsung 1.5 TB drive. I hooked it all up, can boot just fine, but windows doesn't show the drive. BIOS shows it, as does my RAID building software. I have SATA set to RAID in BIOS, and I think that's my problem. I need to set it to SATA 1 and 2 as RAID, but SATA 3 as IDE. How can I do this? Any help would be greatly appreciated.
That's probably not the problem. The problem is probably that the drive isn't formatted yet. Try to find Disk Management in your control panel (it might be labeled as "Create and Format Hard Disk Partitions"), and from there you should be able to see and format the drive.
In RAID systems I've seen, sometimes in BIOS Setup you can designate individually whether each SATA port can be used as a RAID device or not. Others make a blanket assignment - either all SATA ports MIGHT be used for RAID, or none. BUT even after that is set in BIOS, you still have to enter the RAID utility screens during the boot sequence to set up any RAID array or manage it. Within that utility, ONLY drive units specifically assigned to a particular RAID array are used that way. Any not assigned here are by default NOT part of any RAID system. And I'm sure you could not assign a SINGLE disk (your new one) to any RAID array - those arrays always require two disks or more.
Any new disk needs a two-step preparation process before Windows can use it. First you Create a Partition (define a region of the physical HDD, whether all of it or less) to be used as a "disk" with its own letter name. Then you Format it - that is, you install a File System on that Partition to manage the files you write to it. Sometimes these two steps will appear to be combined into one operation by a Wizard which helps to simplify the process. You can get third-party utilities to do this work (including free ones you can download from you HDD manufacturer and install / run on your existing drive), or you can use the tools built into Windows in Disk Management.