Alan, this is what I've found regarding storage capacity of the MX-2424 drives:
The maximum size of a hard drive should be no more than 72GigaBytes external or 36GigaBytes internal when both an internal hard drive and a hot swap internal drive exist. This limit on these two internal drives is because of heat buildup inside the MX-2424. This heat build up can destroy one or both of these drives.
It's not clear whether the 72GB limitation is mandated by the OS or simply guidance to avoid heat issues. In theory, if the latest OS update reads FAT32 drives, then all 300GB should be accessible, but I have no way to confirm that.
Ironically, a 300GB drive is probably both cheaper and easier to find today than a 72GB drive, especially if you go the SATA + adapter route.