Using any 11.0-CURRENT in the last 2 months, the Apple iMac 1024 27" runs EFI boot successfully from the internal SD card bay, but is unable to complete the boot process. The boot loader and kernel are started, but the root fs has not mounted yet.
I don't know how to get a dmesg this early on, but the console displays this repeatedly every few seconds:
Sys addr: 0x00000000 | Version: 0x00001502
Blk size: 0x00000000 | Blk cnt: 0x00000000
Argument: 0x00000000 | Trn mode: 0x00000000
Present: 0x1fff0000 | Host ctl: 0x00000001
Power: 0x0000000f | Blk gap: 0x00000000
Wake-up: 0x00000000 | Clock: 0x0000fa07
Timeout: 0x00000000 | Int stat: 0x00000001
Int enab: 0x01ff00fb | Sig enab: 0x01ff00fb
AC12 err: 0x00000000 | Slot int: 0x00000001
Caps: 0x176ec8b0 | Max curr: 0x00000000
Under 10.1 this system booted successfully from the same device and SD card. Is there any further info I can grab?