Daniel T, I had this exact problem on a Compaq Deskpro EN P550. I recently installed a 200GB hard drive. At first, debian (actually grub) produced the "Error 18". After some research, I found the Compaq BIOS (press <F10> to enter BIOS on boot) had "IDE Translation" (something like that) "Enabled".
This forced the BIOS to only see 6.4GB of my whopping 200GB drive. Of course, this caused the Grub loader to "freak out" because it could not find the kernel files.
The "short and sweet" answer was to DISABLE the "IDE Translation" in the BIOS... all is now well and I am typing this from my new Debian install... GREAT!
Ohhh, by the way, if you receive the "<F1>: Boot" message for any reason, send me an email at
shag_nospam@earthlink.net. I have a fix for that. Unfortunately, I do not have the link on this new Debian install. It is still on my Winbloze laptop.

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Please post back if this helps or send me an email.