Depending on what computer you got but they are found inside the computer or tower.
HDD is an abbreviation for "Hard Disk Drive." Hard disk drives are the primary form of file storage found in a computer.
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.
The usb drives can be found on the side of your key board. They are very easy to be found and are at a convenient location to you on your Mac computer.
There is no such thing as a "Mac" or a "Windows" hard drive. From a technological standpoint the hard drives found in Macintosh computers and those found in computer running Windows are identical. Any hard drive with the corresponding interface (SATA, IDE in older Macs, and most SCSI drives in really old Macs) can be used in either computer. The partition table and file system used by each operating system is generally not readable by the other, but the hard drive itself can be formatted and repurposed.
Hard Drives, CDs, Floppy disks, USB drives.
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.
the hard drives can be switched and what ever was on them will stay on them.