Apparently this is "not a bug" per comments on github where I had attempted to file this, so my post here:
Following the instructions on the site, Mirabox boots fine. However, if at the same time a (non-bootable) external USB disk is plugged in, I get this:
$this->bbcode_second_pass_code('', '
[ 4.237515] VFS: Cannot open root device "sdb2" or unknown-block(8,18): error -123
[ 4.245195] Please append a correct "root=" boot option; here are the available partitions:
[ 4.253647] 1f00 4096 mtdblock0 (driver?)
[ 4.258763] 1f01 4096 mtdblock1 (driver?)
[ 4.263900] 1f02 1040384 mtdblock2 (driver?)
[ 4.269028] 0800 976762584 sda driver: sd
[ 4.273702] 0801 524288000 sda1 20ad5a4e-01
[ 4.278633] Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(8,18)
')
This may be related, earlier in the log:
$this->bbcode_second_pass_code('', 'Hit any key to stop autoboot: 0
(Re)start USB...
USB: Active port: invalid port number 2, switching to port 0
Register 10011 NbrPorts 1
USB EHCI 1.00
scanning bus for devices... 4 USB Device(s) found
Waiting for storage device(s) to settle before scanning...
scanning bus for storage devices... Device NOT ready
Request Sense returned 02 3A 00
2 Storage Device(s) found
')
Also, even if the external HD is not plugged in, I notice that my SDCard comes up as /dev/sdb, so /etc/fstab fails to mount /boot because it is looking for /dev/sda1 instead of /dev/sdb1. As a result, the boot sequence drops into maintenance mode. Changing /etc/fstab and rebooting fixes this.