I am new to Open Source. To prove it, I am confused about hard drives. I see so many references to multiple hard drives, but manufacturers websites only speak of external hard drives as either storage or back-up devices. All the Open Source information I have read makes me think that an external hard drive in a USB can have a bootable OS on it. Can someone enlighten me?
Most motherboards that I've looked at recently allow from 4 to 8 internal SATA drives plus 2 IDE internal drives, whether they be hard disk or optical drives. Not sure whence you got your limit of just 4.
^ "recently" is your keyword here.
Back in the days of the (P)ATA ( sounds epic ), computers usually only had two controllers/cables for optical/hard disks, with a master and a slave for each, i.e. 4 disks.