Yes, provided that the power supply is big enough to handle the extra drain AND provided that you have enough space and connections. Those restrictions are all based on what the manufacturer put in the computer.
hot swappable hard drives are drives that can be taken out and replaced while the computer is running. mainly found on servers and network attached storaged devices. All external usb/firewire drives are hot swappable in that you can unplug an external and plug in another external without a problem. This term could also refer to an external hard drive that has hot-swappable drives. For example, you can pull the hard drives out of the external replace them on the fly.
In the computer's HARD
The most common problem with large hard drives is computer crashes.
Assuming that when you say system unit of a computer you mean the computer case A.K.A. the tower, then no, hard drives are not always installed internally. Hard drives can be in external enclosures, hard drive docks, and even network attached devices.
The best place to visit about purchasing computer hard drives is an online website. A person can go on Amazon, Ebay, Newegg, or Bestbuy website to find the best place to visit about purchasing computer hard drives.
hard drives
The space inside the computer for DVD/CD/Floppy(who uses these anymore?) Drives and also Solid State Drives (SSD) and Hard Drives.
Depending on what computer you got but they are found inside the computer or tower.
Hard Drives, CDs, Floppy disks, USB drives.
Internal Desktop refers to electronic computer parts that are designed to be installed inside a desktop computer. e.g Internal Desktop Hard Drives are hard drives designed to be installed inside a desktop computer.
the hard drives can be switched and what ever was on them will stay on them.
This company sells used computer equipment. The equipment can include computers, networking equipment, computer components like hard drives and disc drives.