Problem Starting X

Excuse me posting a slightly off-the-wall issue here, but I feel fundamentally it is a Slackware problem.

On my Raspberry Pi 4, I have installed Slarm64. It's an unofficial compile of Slackware packages for Arm64bit sbcs generally. The RazPi 4 booting order is

1. SD Card
2. USB disks

Now if I put an sdcard in, X boots fine, and things work. It's set to boot to runlevel 4. I also rsync'ed the sdcard partitions to a real disk, and adjusted boot location & fstab accordingly, and that boots to runlevel 3. Then it says
Starting X11 Session Manager ...
Starting X11 Session Manager ...
Starting X11 Session Manager ...

repeating that line every 10 seconds. It never starts X. The disk is rsync'ed to the sd card, so it should start. I tried Code:grep -r mmcblk0 /etc/* but it only showed in /etc/mtab (which is strange, actually, because there was no sdcard in it). /var/log/Xorg.0.log was from the previous boot of X.

Can someone point me to the script that's run to start runlevel 4 or employ their superior knowledge of boot procedures to diagnose my issue?