I am using Slackware 8.1 and I have this problem with disk size.
------------------------------------------------------------------------

user@xxx: dmesg

hde: Maxtor 32049H2, ATA DISK Drive
hde: 40021632 sectors (20491 MB) w/2048Kib Cache, CHS=39704/16/63, UDMA(100)

Partition Check:

hde: [PTBL] [2491/255/63] hde1 hde2

------------------------------------------------------------------------
user@xxx: df /dev/hde
FileSystem 1k-blocks Used Available Use% Mounted on
/dev/hde 4964188 4263704 444248 91%

------------------------------------------------------------------------
user@xxx: fdisk /dev/hde

Device Boot Start End Blocks Id System
/dev/hde1 1 33 265041 82 Linux swap
/dev/hde2 * 34 2491 19743885 83 Linux

-------------------------------------------------------------------------

during the initial boot, the disk is recognized correctly as 20GB. Later on disk free command
shows only 4.7G!

I am at a loss as the hard disk is filling up fast!

Has anyone an idea of what is going on here?

Thanks in advance

Your hde2 partition goes from block 34 to 2491, this is not too much. And the rest of your disk is not partitioned. Change it to 20491 (last block on disk) as end block, or add another partition at the end.
I cannot garantie anything, as I've never done things like that. In every case, do a backup before changing the size of a partition.