I just tried this a couple of ways with an RDB image that I know works fine. 3.2GB total, split into 2 partitions.
I tried by creating a custom system and through the "import hard disk file" option.
Both times, once the image finished copying and I configured the options, when I pressed "Save", the A600GS menu quit out and went back to the "Press any key or button to start" screen.
Neither of the systems I created appear in the list

Let me know if there's any other information I could provide that would be helpful to debug.
Edit: Actually, I noticed that they are in the menu, they just didn't save the name (or any other options) that I set so they're called "Custom_System_3" and "cf20230312" which is the name of the file I imported. I tried editing again but the menu just crashes out again when I hit Save.