2 Workarounds:
to read a flat.vmdk when dd if=name-flat.vmdk bs=1M ... fails
1. use a helper VM and create a protective snapshot added to the original basedisk.
2. dont access the vmdk-file - acess the device
instead of
dd if=name-flat.vmdk bs=1 count=4096 skip=0 | hexdump -C #fails
run
dd if=/dev/disks/mpx.vmhba0:C0:T4:L0:2 bs=1 count=4096 skip=86991962112| hexdump -C #works