not able to flash u-boot onto SD Card

[consoleadam]

FIXED
the trick was to re-create the paritions after dd'ing, as following:

making dummy system.img (of size 2000 MB)
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consoleadam@barbarian:~/rock64$ dd status=progress if=/dev/zero of=system.img bs=1M count=0 seek=2000 status=none

adding my custom built images to the dummy image
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consoleadam@barbarian:~/rock64$ umount /media/consoleadam/*
consoleadam@barbarian:~/rock64$ dd status=progress if=idbloader.img of=system.img seek=64 conv=fsync status=none
consoleadam@barbarian:~/rock64$ dd status=progress if=uboot.img of=system.img seek=16384 conv=fsync status=none
consoleadam@barbarian:~/rock64$ dd status=progress if=rk3328evb-trust.img of=system.img seek=24576 conv=fsync status=none

you can add kernel/rootfs images to dummy system.img from another built image as following:
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consoleadam@barbarian:~/rock64$ dd status=progress if=jessie-openmediavault-rock64-0.5.10-118-arm64.img of=system.img seek=32768 skip=32768 count=229376 conv=fsync status=none
consoleadam@barbarian:~/rock64$ dd status=progress if=jessie-openmediavault-rock64-0.5.10-118-arm64.img of=system.img seek=262144 skip=262144 count=4194304 conv=fsync status=none
consoleadam@barbarian:~/rock64$ sync

re-partitioning the dummy image
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consoleadam@barbarian:~/rock64$ parted -s system.img mklabel gpt
consoleadam@barbarian:~/rock64$ parted -s system.img unit s mkpart loader1 64 8063
consoleadam@barbarian:~/rock64$ parted -s system.img unit s mkpart reserved1 8064 8191
consoleadam@barbarian:~/rock64$ parted -s system.img unit s mkpart reserved2 8192 16383
consoleadam@barbarian:~/rock64$ parted -s system.img unit s mkpart loader2 16384 24575
consoleadam@barbarian:~/rock64$ parted -s system.img unit s mkpart atf 24576 32767
consoleadam@barbarian:~/rock64$ parted -s system.img unit s mkpart reserved3 32768 262143
consoleadam@barbarian:~/rock64$ parted -s system.img unit s mkpart root 262144 100%
consoleadam@barbarian:~/rock64$ parted -s system.img set 7 boot on

below in one go:
-----------------

consoleadam@barbarian:~/rock64$ parted -s system.img mklabel gpt; \
> sync; \
> parted -s system.img unit s mkpart loader1 64 8063; \
> sync; \
> parted -s system.img unit s mkpart reserved1 8064 8191; \
> sync; \
> parted -s system.img unit s mkpart reserved2 8192 16383; \
> sync; \
> parted -s system.img unit s mkpart loader2 16384 24575; \
> sync; \
> parted -s system.img unit s mkpart atf 24576 32767; \
> sync; \
> parted -s system.img unit s mkpart reserved3 32768 262143; \
> parted -s system.img unit s mkpart root 262144 100%; \
> sync;\
> parted -s system.img set 7 boot on


burning the dummy image to SDcard
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consoleadam@barbarian:~/rock64$ sudo dd status=progress if=system.img of=/dev/sdb conv=fsync

Note that you can only dd specific parts [dd status=progress if=idbloader.img of=system.img seek=64 conv=fsync status=none] directly to SDcard instead of to dummy image (system.img) but also make sure to repartition the SDcard after dd'ing!