Why won't my Fujitsu HDD 8.4 GB IDE not fully format?

Have you tried something like fdisk to delete the partitions that are active and then tried to format it? There is some low level formatting software out http://hddguru.com/software/2006.04.12-H... Now if you are an old DOS person you could even try to use Debug for this :-)and Microsofteven has it now on their site

1. Start DEBUG by typing the following at the command prompt:

debug

2. At the DEBUG hyphen (-) prompt, type one of the following commands:

G=C800:5

G=CA00:5

G=CC00:5

G=CE00:5

The above commands execute instructions starting at either location C800:0005, CA00:0005, CC00:0005, or CE00:0005, depending on which address is selected. If the computer freezes with the first address listed above, try the second address.

The program in the controller's ROM will execute and will query for interleave factor and other parameters. Answer these as appropriate; the hard disk will then be low-level formatted.

Give it a try and see what happens. BTW what OS are you using and what computer are we talking about?