When you did this:
- gparted /dev/sdc, then resized partition to 750 GB
you got a nice ~750 GB partition,
but when you did this
- ddrescue /mnt/c/temp/original-mbr.bin /dev/sdc
you restored the definition of the original ~250Gb partition.
i think you should do this:
- install the 750 GB disk in the Asus, and install the firmware on it.
- take it out, and make the partition visible. (backup the MBR first)
- make the partition on the 250GB visible, if needed.
- copy the the files someway (cp)
- restore the mbr of the 750GB disk
But I have to say that i never tried to transfer my files to a bigger disk. So I hope for you that Robin or Omega24 will reply with a solution that has proven to work.
Marc