First…you need to modify /boot/cmdline.txt to point to the root partition of the SSD/HDD. It will probably be /dev/sda2. Next, you should change fstab to mount the /dev/dsa partitions. The full instructions are here: Raspberry Pi Documentation - Raspberry Pi hardware
Basically, you need both /boot and /root partitions, with the /boot being a FAT32 file system.
As for the delay…that’s because the VC4 is looking for an SD card or eMMC with a boot partition and the right file in it. When that isn’t found after 5 seconds, it looks for a USB attached mass storage device. There is a way to shorten the interval, but I haven’t bothered as I don’t spend lots of time rebooting Pis (and if I do, I’ll live with it on an MSD setup).
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Thanks a lot. I’ll try it later this week- I’m on the road for a couple of days. I did modify cmdline.txt but may have forgotten fstab.
I have a CM3 and this SATA Adapter… It tanks for my use case. I cannot create a ticket because I cannot find the SN for the device…
It was hard to even format the drive, then it semi-worked under low use but froze under load. Happened with both a 2TB SSHD and a 240GB SSD. When it happened with the SSD it corrupted the partition table of the device.
Can someone help?
My original SATA Asapter works find. I just set the boot from USB OTP bit and swapped in a CM3L. That one was purchased when they first came out. Of my two newer ones, one doesn’t work at all and the otehr will work with a CM running from the on board eMMC flash, but not with an SSD attached with either the CM or the CM3L.
This is all using a 2.4A PSU.
So…dunno. It looks like something changed at some point. It’s probably all moot, anyway, as WD Labs is going away and so are the WD Pi products. Pity, really…
I really wish I knew that before I purchased the device 
I’m using a Arctic Smart Charger 8000, as the PSU so I don’t think its on the PSU side.
I also can’t seem to reflash the EMMC on the CM3 again.
