Thanks a lot for those who tried to help me. Here is the history of what
happened next.
First, to follow Ben's suggestion, I yesterday downloaded Max Power and
tested the problem disk. To my surprise, it passed all the test and thus
it appears the disk does not bear any problem as far as Maxtor Power was
concerned.
Secondly, to test Don's suggestion, when I used it as the primary, the
computer BIOS did not recognize it.and failed to boot.(I also tested it
with another computer but the result is the same.)
When I used floppy disk to boot the computer followed by FDISK, it
responded that there is no hard drive. Accordingly I am unable to
examine its partion status.

Today, when I booted the computer with anothe drive as the primary and
the problem disk as a slave, the BIOS recognized the primary disk as
expected but did not detect the slave. This is a different situation
from the time when I posted initially when BIOS recognized both drives
and later the device manager listed both as working properly. It appears
the problem somewhat worsened since then.
After the above setting the computer booted to Windows, but Windows did
not recognize the problem disk as judged by Windows Explorer obviously
because the device manager now did not list it, either.

Now I wonder if there is any way to correct the problem without
reformatting the problem disk. I do not even know if reformatting is
possible because floppy-booted computer does not recognize the presence
of this disk. As the last resort I tried the Maxtor Plus installation
disk, but it conveyed that there is no harddrive. In my long computer
experiences, I never encountered a situation like this.
If anyone still can offer suggestions, I would really appreciate it.