Both internal and external hard drives can be removed from a computer provided that the PC does not boot from any of them. In that case, the first boot device would have to be a removable drive like CD-ROM/DVD-ROM or floppy drive.
the hard disk
System unit
Floppy drives and CD/DVD roms are examples of removable disks because the storage medium is removed from the drive. A hard drive has a set of magnetic platters that you do not remove, making it a fixed disk.
Sector is the smallest unit on Hard disk identified & addressed by File System.
Hard Drive
external hard drive
Yes they are, although you can get external hard drives that are separate.
the purpose of input unit is to input datas or information into the system
If by "system unit," you mean the main part of the computer, then yes. In the past, all disk controllers were plugged into the motherboard. Nowadays, the motherboards all come with disk controllers, at least for SATA (and sometimes SAS) hard drives.
The hardware unit that contains the CPU, the RAM, a hard disk, and a power supply is called the "motherboard."
Hard Drive Disk (HDD)
Fixed disk