From what I have read before, the LiteOn 5045 uses FAT32 on its internal hard drive. The only thing I am not sure about is if they have any separate parition or section of the hard drive reserved for storing the recorder's software or if the firmware is specific to the exact hard drive shipped with the recorder. However, as there are LiteOn models without a hard drive (such as the 5005), there is a good chance that the hard drive is strictly used for recordings.
What you can try is connecting up a hard drive without any partition on it. Most PVR's will display a message saying something like "Hard drive data is corrupt. Do you want to format?". If this is the case, just proceed with the format and the hard drive will be initialised and should work fine. However, if you get a blank screen or a warning that the hard drive is unrecognised or corrupt without any option to format, then another thing you could try is to make a replica of the original hard drive onto this. In this case, partition the hard drive with a basic primary partition and format it in FAT32. Then make a full copy of the original LiteOn hard drive onto this including all hidden and system files (if any).
As the firmware is intended to be used with the original hard drive, there is a chance that the firmware will only work with a matching 160GB hard drive, so I would recommend going for 160GB hard drive of the same brand and model if you can.