You probably installed a custom MIOS (which is not the same as DIOS MIOS) since this was the way to play backed up GameCube Games from burned discs back in the day. Now you have three options to choose from:
1. if you want to be able to ONLY play original GameCube games again, all you have to do is install a clean MIOSv10, which can be obtained via
ModMii. Then, get a wad manager (also available through ModMii if you don't have one at hand) and install the clean MIOSv10 with it. There you go, originals, but not backups, should be working again. If not, then you probably have to install a clean BCv6 as well. Just download it with ModMii like the MIOSv10 and install it with the wad manager.
2. Then there is the option of a more modern setup with DIOS MIOS (Lite), which is able to play GameCube backups either from an external HDD or the SD Card and you should also be able to stick an original disc in the wii and play it with DIOS MIOS installed.
DIOS MIOS (Lite) can be obtained on the
official google code page. However, before installing DIOS MIOS I would install a clean MIOS like described in option 1. Then, just download the DIOS MIOS version you prefer (the normal DIOS MIOS for loading games from the HDD, and the Lite Version for loading games off SD Card, but always the version for real nand) and install the wad.
If you choose the HDD route, it can have more than one partition, but the first partition needs to be FAT32 formatted with a clustersize of 32kb and set to primary & active. This can be done with a partition manager such as
EASEUS Partition Master, but mind that you have to back up important data from the HDD before you format it, otherwise all data is lost. To finally play the games, just get one of the common usb loaders like wiiflow or usbloader gx and you're set.
3. Then There is also a tool called devolution, which is also able to load games from usb, but has some advantages over DIOS MIOS like working audio for some games that stream their music and other sounds directly from the disc, something DIOS MIOS does not have. There are also things like BBA emulation to play games like Phantasy Star I and II online and classic controller support as well as GCN to GBA Link emulation is planned.
To be able to use it you will need to verify the games you want to play from the HDD with the original disc once (the tool prompts you to insert the original disk the first time you start it from the HDD to verify it)
Since I'm a DIOS MIOS user, I cannot give any instructions on how to install devolution, but
here should be everything you need to know.