U-Boot fails to load from SD when a USB dual HDD device is attached

I'm running FreeBSD 13.1 on the Raspberry Pi 4B, with a new dual 
SATA-to-USB hard drive docking station, and I've noticed a peculiar 
situation: U-Boot fails to load FreeBSD from the SD card when the 
docking station is plugged in and has two hard drives in it (presenting 
two USB storage devices to U-Boot), but still succeeds when:

 1. the device is not attached;
 2. the device is attached but only has one hard drive in it, presenting
    one USB storage device to U-Boot;
 3. when the device with one hard drive in it, and another USB storage
    device, such as a flash drive, are plugged in.

Any clue what might be causing this problem? I wonder if this might be 
an issue with voltage sag/over-voltage on the USB interface causing the 
SD card not to select, or something similar, although the docking 
station has its own power supply and so should not be drawing a 
substantial current from the pi.

I've included what errors I collected through the serial interface.


    U-Boot 2021.07 (May 12 2022 - 07:00:33 +0000)

    DRAM:  3.9 GiB
    RPI 4 Model B (0xc03112)
    MMC:   mmc@7e300000: 3, emmc2@7e340000: 0
    Loading Environment from FAT... In:    serial
    Out:   vidconsole
    Err:   vidconsole
    Net:   eth0: ethernet@7d580000
    PCIe BRCM: link up, 5.0 Gbps x1 (SSC)
    starting USB...
    Bus xhci_pci: Register 5000420 NbrPorts 5
    Starting the controller
    USB XHCI 1.00
    scanning bus xhci_pci for devices... 3 USB Device(s) found
            scanning usb for storage devices... 2 Storage Device(s) found
    Hit any key to stop autoboot:  0
    U-Boot> boot
    switch to partitions #0, OK
    mmc0 is current device
    Scanning mmc 0:1...
    libfdt fdt_check_header(): FDT_ERR_BADMAGIC
    Scanning disk mmc@7e300000.blk...
    Disk mmc@7e300000.blk not ready
    Scanning disk emmc2@7e340000.blk...
    ** Unrecognized filesystem type **

    ...

    run bootcmd
    switch to partitions #0, OK
    mmc0 is current device
    Couldn't find partition dhcp 0:1
    MMC Device 1 not found
    no mmc device at slot 1
    MMC Device 2 not found
    no mmc device at slot 2

    Device 0: Vendor: ASMT     Rev: 0    Prod: ASM1156-PM
                 Type: Hard Disk
                 Capacity: 305245.3 MB = 298.0 GB (625142448 x 512)
    ... is now current device
    Couldn't find partition dhcp 0:1
    BOOTP broadcast 1
    BOOTP broadcast 2
    DHCP client bound to address 192.168.1.33 (1246 ms)
    *** ERROR: `serverip' not set
    Cannot autoload with TFTPGET
    missing environment variable: pxeuuid
    Retrieving file: pxelinux.cfg/01-dc-a6-32-90-1f-1e
    *** ERROR: `serverip' not set
    Retrieving file: pxelinux.cfg/C0A80121
    *** ERROR: `serverip' not set
    Retrieving file: pxelinux.cfg/C0A8012
    *** ERROR: `serverip' not set
    Retrieving file: pxelinux.cfg/C0A801
    *** ERROR: `serverip' not set
    Retrieving file: pxelinux.cfg/C0A80
    *** ERROR: `serverip' not set
    Retrieving file: pxelinux.cfg/C0A8
    *** ERROR: `serverip' not set
    Retrieving file: pxelinux.cfg/C0A
    *** ERROR: `serverip' not set
    Retrieving file: pxelinux.cfg/C0
    *** ERROR: `serverip' not set
    Retrieving file: pxelinux.cfg/C
    *** ERROR: `serverip' not set
    Retrieving file: pxelinux.cfg/default-arm-bcm283x-rpi
    *** ERROR: `serverip' not set
    Retrieving file: pxelinux.cfg/default-arm-bcm283x
    *** ERROR: `serverip' not set
    Retrieving file: pxelinux.cfg/default-arm
    *** ERROR: `serverip' not set
    Retrieving file: pxelinux.cfg/default
    *** ERROR: `serverip' not set
    Config file not found
    BOOTP broadcast 1
    BOOTP broadcast 2
    DHCP client bound to address 192.168.1.33 (907 ms)
    *** ERROR: `serverip' not set
    Cannot autoload with TFTPGET
    BOOTP broadcast 1
    BOOTP broadcast 2
    DHCP client bound to address 192.168.1.33 (1312 ms)
    *** ERROR: `serverip' not set
    Cannot autoload with TFTPGET
    U-Boot>


Thanks!

Bradley