No
While external hard drives have the availability to work with multiple computers, it would be false to say every external hard drive works with every computer.
If you want to synchronize files to multiple computers with an external hard drive or flash drive, you can do that. It is called a briefcase. It automatically syncs and updates files on multiple computers that you choose. If you want some more information on what briefcases can do, go to the related link below.
Only if you copy them.
An external hard drive is a device similar to your computer's hard drive. The difference is that it is portable. You can save files on it and it can be connected to most other computers.
IDE 0 always has the designation C. Its a throwback to the days when computers were single or dual floppy drive only and they had (and still have) designations A and B. Hook up the external drive, go into the BIOS and select the external as the drive to boot from. Save the change and allow the computer to boot. Load the OS and it should defer to the external drive.
I presume you are asking if you can use an external drive on different computers? The simple answer is yes but you can't install programs on the external drive to run on other computers (except the one it was installed on), this is because programs spread themselves in different parts of the computer, so bits of the programs would be missing if you tried to run it on another computer. Other than this the only problem is viruses which can be transmitted through infected files if you aren't careful where you plug in. NOTE the external drive "could /will" change its drive letter depending how many drives are on the host computer this isn't a problem! just be ready to look for your files in a different labelled drive in "My Computer" (presuming you are using a windows computer)
if you purchase special software this will be possible.
Yes absolutely! Most external hard drives have support for as many computers as youd like. just plug-in and go.
yes they do read disk. some require an external drive.
Computers that have multiple DVD drives in them are typically used for recording or copying multiple CDs or DVDs at once.
Only if you can boot from it. Most new computers can boot from a USB drive, but older ones may not be able to.
If a hard drive is external to computer then it is easy to move it between computers if required or take it away for safe keeping. If it has a good had drive enclosure then it is more protected from the elements or from accidental damage.