OpenCore install boots to drive select, even though only one boot device available

Hi

This might not strictly be an OpenCore problem, so apologies if so. I've installed OpenCore (Ventura) on an iMac 27" (iMac 13,2). The slight complication is that the Fusion drive inside the iMac is as good as dead, so I'm booting off a SSD that is physically attached to the back of the iMac.

This has worked fine when I was using regular macOS, but after installing OpenCore (which turned out to be quite problematic - I had to hold down Control every time it booted during the install process to manually select the EFI option) it is all working, but every time I boot the iMac it pops up with the drive selection page, except there is only one option available - the external drive. if I don't press anything it carries on booting and everything works normally from then on.

I've checked the "Startup Disk" and its set to the external drive.

It's only a minor annoyance but I'd like to fix it if I can. Any ideas? Did I do something wrong during the install?

Thanks in advance!