2 HDD/ Drive letter problem....WHY??

Wrong - Unless the whole drive is unallocated space

Windows will Install and Boot to the drive you specify in system BIOS and only to the Active Partition which will be labeled C:\. All other drives will be labeled sequentially as found. If your Windows files and Boot files are both on C:\, all other drive letters can be changed through Disk Management found in Computer Management.

Wow dude.. calling me out like that is wrong. Especially when I phrased it correctly with generally.
Windows will almost always try to install to the first drive partition it sees and give preference to an unallocated drive space. Potatoe/potahto.

I didn't call you out, I called out the bad terminology (unallocated).

Windows does not look for unallocated space to call home. If this was the case you would have trouble installing to any previously created partition. Windows looks for all space allocated and unallocated and calls home the space you specify for the installation.