The CF slot is wired for master cable select (CSEL), you can install a hard drive if you have jumpers on the drive to make it cable select and the hard drive will be slave instead of master. CSEL involves a line 28 of the 40 or 44 pin IDE cable to either be grounded or +5 volts, depending on the charge the device is either slave (ground) or master (+5volts); this is usually involved in either the IDE interface or the a modified cable. I made such a special cable to connect a laptop cdrom drive and IDE hard drive togather since the cdrom drive I was using was set to use CSEL only and could not be configured by jumpers as a slave device. The big issue is that you would need to cut the line 28 on the standard IDE interface and line 38 on the CF interface from it's source were ever it's getting either the ground or +5 volts WARNING!!! if you don't, you will fry the board since you will putting 5 volts directly to ground.
I actually modified one GCT mainboard to use the CF as a slave IDE device, the problem is that my mod was a fluke and the resister that I pulled wasn't actually the one to make it work as I must have disturbed some other line since I have not been able to reproduce my results. I believe line 38 of the CF is the CSEL line, if you can pull it's +5 volt line and ground it you should be able to make the CF socket slave; of course you will need to do the same to the IDE 44 interface and you make sure that line 28 isn't grounded and put it to +5 volts. The draw back is that once such a mod is preformed you will not be able to boot on the CF again unless you can reverse the mod or install a switch or jumper.
Even if you are succesful, the CF is not hot swappable as the GCT must be powered down to change or take out the card. A $29 USB reader serves the purpose much better to allow the removable storage of the CF card (and much safer than the chance of burning up your GCT). you should be able to get one ofr less than $29 but that's what I paid last year for a Sandisk USB reader.