Followed the instructions and wrote the image to an 8GB SD card using balenaEtcher (even tried to verify the image). But it fails to boot.
...
[ 3.505879] hidraw: raw HID events driver (C) Jiri Kosina
[ 3.511781] drop_monitor: Initializing network drop monitor service
[ 3.518404] TCP: cubic registered
[ 3.521719] NET: Registered protocol family 17
[ 3.526375] Key type dns_resolver registered
[ 3.531726] Loading compiled-in X.509 certificates
[ 3.536583] registered taskstats version 1
[ 3.542156] drivers/rtc/hctosys.c: unable to open rtc device (rtc0)
[ 3.550790] VFS: Cannot open root device "(null)" or unknown-block(0,0): error -6
[ 3.558348] Please append a correct "root=" boot option; here are the available partitions:
[ 3.566723] Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,0)
[ 3.574962] CPU: 0 PID: 1 Comm: swapper Not tainted 3.18.5-kirkwood-tld-1 #1
[ 3.582020] [<c0014aa0>] (unwind_backtrace) from [<c00110d4>] (show_stack+0x10/0x14)
[ 3.589747] [<c00110d4>] (show_stack) from [<c054f408>] (panic+0x80/0x208)
[ 3.596606] [<c054f408>] (panic) from [<c076c200>] (mount_block_root+0x200/0x248)
[ 3.604065] [<c076c200>] (mount_block_root) from [<c076c32c>] (mount_root+0xe4/0x10c)
[ 3.611866] [<c076c32c>] (mount_root) from [<c076c478>] (prepare_namespace+0x124/0x184)
[ 3.619841] [<c076c478>] (prepare_namespace) from [<c076be2c>] (kernel_init_freeable+0x1f8/0x240)
[ 3.628679] [<c076be2c>] (kernel_init_freeable) from [<c054e500>] (kernel_init+0x8/0xe4)
[ 3.636741] [<c054e500>] (kernel_init) from [<c000e310>] (ret_from_fork+0x14/0x24)
[ 3.644283] ---[ end Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,0)
U-Boot 2016.05-tld-1 (Jun 12 2016 - 13:41:47 -0700)
Pogoplug V4
gcc (Debian 4.9.2-10) 4.9.2
GNU ld (GNU Binutils for Debian) 2.25
Hit any key to stop autoboot: 0
MMC rescan: current device # 0 initialized OK
starting USB...
USB0: USB EHCI 1.00
scanning bus 0 for devices... 1 USB Device(s) found
scanning usb for storage devices... 0 Storage Device(s) found
Reset IDE: ide_preinit failed
Partition Map for MMC device 0 -- Partition Type: DOS
Part Start Sector Num Sectors UUID Type
1 1 7792469 00000000-01 83 Boot
loading envs from mmc 0 ...
** File not found /boot/uEnv.txt **
no USB devices available
no IDE devices available
running scan_disk ...
Scan device mmc
MMC rescan: current device # 0 initialized OK
device mmc 0:1
1 bytes read in 713 ms (0 Bytes/s)
Found bootable drive on mmc 0
loading uImage ...
2875692 bytes read in 1197 ms (2.3 MiB/s)
loading uInitrd ...
6535284 bytes read in 1402 ms (4.4 MiB/s)
loading DTB /boot/dts/kirkwood-pogoplug_v4.dtb ...
10028 bytes read in 2901 ms (2.9 KiB/s)
## Booting kernel from Legacy Image at 00800000 ...
Image Name: Linux-3.18.5-kirkwood-tld-1
Created: 2015-11-11 5:03:31 UTC
Image Type: ARM Linux Kernel Image (uncompressed)
Data Size: 2875628 Bytes = 2.7 MiB
Load Address: 00008000
Entry Point: 00008000
Verifying Checksum ... OK
## Loading init Ramdisk from Legacy Image at 01100000 ...
Image Name: initramfs-3.18.5-kirkwood-tld-1
Created: 2015-02-19 1:49:36 UTC
Image Type: ARM Linux RAMDisk Image (gzip compressed)
Data Size: 6535220 Bytes = 6.2 MiB
Load Address: 00000000
Entry Point: 00000000
Verifying Checksum ... OK
## Flattened Device Tree blob at 01c00000
Booting using the fdt blob at 0x1c00000
Starting kernel ...