The first thing I would do is to get and use a dry, brush-type optical drive cleaning disk. I have this one:
Very Mac-friendly in its operation. Often a seemingly dead optical drive can be restored to full function with a cleaning. The disks are certainly cheaper than most external drives.
I suspect that the external optical drive has a mechanical and/or electronic problem, as opposed to having a compatibility issue with your G5. I've got many different external optical drives/burners (by a variety of manufacturers) and the only problem (if any) with a Mac would be bootability. Reading disks is not typically a problem, and burning disks (with "Toast") is trouble-free. Other World Computing has this external DVD±RW drive, which has every connection option you'd need. If you want a Blu-ray drive, there are a several from which to choose on this page.
There is no way (or need) to disable the internal optical drive with a control panel, as one might do with the Device Manager in a PC or with BIOS settings. The compatibility of the OEM internal drive doesn't diminish the System's capability to recognize an external drive.