First, try the external drive in some other "recent" computer.
Use the other computer to "format" the disk-space,
and thus make an "empty" partition. Then, reconnect it to your computer, and see if the total capacity of the drive has been detected.
According to the manufacturer's web-site:
http://h20000.www2.hp.com/bizsupport/TechSupport/SoftwareIndex.jsp?lang=en&cc=us&prodNameId=96717&prodSeriesId=96231&swLang=8&taskId=135&swEnvOID=1093
this computer dates from 2001/2002. So, the claim of a maximum of 32GB probably is correct -- drives larger than 32GB were either not-available, or were prohibitively-expense, nearly 10 years ago.
Download the latest BIOS-update from the above web-site, and CAREFULLY follow the instructions to update the BIOS.
However, you have not said what happens when you connect the external disk-drive, as a USB device. It *MIGHT* work "as is", i.e., without trying to update the BIOS.