I assume you mean floppy disk. Hard disks (hard-drives) are designed to be connected and not usually removable without opening the case.
if you are referring to a Zip drives' hard-disk then no - not usually - unless it is required by an application or as a security measure.
True.
They either use a rescue disk or they insert the hard disk into another computer as a secondary drive.
A computer hard disk or hard drive (fixed disk) is hardware.
The computer's hard disk is hardware.
Simply connect your external hard disk to your computer. Then select the songs you want to play later, copy them and paste to your computer hard drive. And after disconnecting your external hard disk you can now play the selected songs from your computer hard drive.
The OS is stored in the Hard Disk of the computer.
The best way, I think to answer that is to define what a computer (or in this case the motherboard of the computer) needs to find or identify a hard disk. A hard disk needs a power source, a connector to attach it to the motherboard, and a controller to talk to the microprocessor of the computer. If any of these things fail the hard disk will not function and the computer will not recognize it. In general when you see the message "hard disk not found" its because the computer can't recognize the hard disk. This can be from a bad cable (that connects the hard disk to the motherboard), no power to the hard disk, or a failed controller. In my experience, its usually a bad cable but there are times when the hard disk itself will fail. Generally, hard disk cables are far more fragile than hard disks.
An external harddrive? Or Memory Stick?? its a removable hard disk
to make the computer function
a hard drive is built into the computer whereas a hard disk is seperate
depends on the computer
It depends on how damaged your hard disk is. There are some instances where there is nothing you can do to fix what's wrong with the hard disk. Other times you may have to insert you operating system disk to initiate a recovery.