There is someone on Reverb offering a D12 with a 40G hard disk.
Therefore, it is going to be an ATA 40G drive. Probably 2.5".
Open it up, and take a look. A higher capacity drive may also work, but might only be seen as 40G.
Your fault could be due to loose cables, so try making sure nothing's worked loose inside.
You can also download the manual, which may tell you all.
Also, there may be a backup battery inside, which needs replacing.
I doubt something that ancient would have an SSD drive, or SATA.