what modchip do you have in it? (does it have anything external on the xbox that you can see on it's case etc?) (im hoping it's a "Xecuter" brand modchip since im most familiar with those... but even if it's not a Xecuter brand modchip odds are it's running some version of the X2.49xx or X2.5032 or X2.5035 bios since those are pretty much 'the standard' for modchips that use 256KB/512KB bios's... the Xecuter3 modchip can use those bios's to but it has it's own X3.xxxx (i forgot version numbers) bios which is fancier than X2's in the sense you dont need bootable cd's in order to get the system up and running etc etc etc)
cause telling me that might give me a ball park figure on what BIOS it's running etc... but since you said it was going to the standard xbox logo (i.e. basically stock) my guess is the system is not even booting the hacked bios. (i.e. might need to adjust the dip switches possibly on it)
but since you said it's got a 500GB hard drive in it AND it boots like a stock xbox does... the hard drive must be locked to the system otherwise if the modchip was off it would error out since the stock system bios requires the hard drive be locked otherwise it will goto a error screen.
but basically... i need more info in order to give you some suggestions
p.s. i got a modchip in mine to (which i put in back around late 2004) and i also have 2 other xbox's i softmoded (i.e. no modchip but does pretty much the same thing a modchip does and best of all it's 100percent free (assuming u got a PC with a IDE connector on it and a cd-dvd burner with a cd-r or cd-rw etc) )