I agree with motorollin, since 1.2 ROM Amigas don't accept ROM upgrades very well.
Open the A500 and see what ROM version you have. I bet you have some sort of 2.0x on it.
Easy to check: power up your Amiga connected to a TV or monitor with no diskette inside the unit and look at the initial screen.
If 1.2 or 1.3, a "hand" appears with the ROM version on the disk figure held.
If 2.0x the boot screen is totally different! You'll see a disk drive on the right and the exact ROM version on the left. Normal range is 37.199 to 37.350 . The higher version, the better, although A500 don't accept IDE HD inside (not without a custom, DIY, IDE controller).