HELP! Multiple hard drives - Fresh install

Fixing a friends older PC. There are two physical hard drives in the computer and I am going to completely format and reinstall Windows10 due to the 1803 failure on that machine (can't get into literally anything now).

So, the PC has 2 hard drives but boots from the smallest one. I want to format and install on the larger drive this time. Can I simply pick the larger drive as the install drive and it becomes the correct disc and boots correctly.

Does the MBR go on the correct drive and make it the bootable drive. Yes, I will fix the BIOS accordingly.

Is that all it takes and the drives do not need to be taken out and physically reversed? IOW, the boot becomes the hard drive no matter where it is connected with the install pointed to it and the BIOS is changed?

Thanks

The drives do not need to be physically removed and reversed. What I would recommend is that you open the case, plug the large hard drive into SATA port 1 on the motherboard. Leave the second hard drive disconnected. Wipe the connected hard drive (delete all the partitions on it), and install Windows to the unallocated space. Then after Windows is installed, connect the second hard drive to SATA port 2. You only have to unplug the cables from the motherboard, you don't have to physically remove the hard drives and reverse them.

You can use the Windows install to delete all the partitions and setup all the needed partitions. The reason to unplug the second drive is that Windows has been known to install the boot loader to the wrong drive.

To add, a number of motherboards will have the first SATA port marked as SATA 0 or Drive/Disk 0, some may start as SATA1, just have to pay attention. Windows Disk Management does the same, Disk 0. Additional drives will be consecutive, Disk 1 is the second, Disk 2 is the third, etc.