@fastrene:
Large files: Make multi-volume (=multi-disk) archive files (.lha .zip .whatever). Slow, but works.
paralell cable: PC2Amiga from aminet should be pretty easy to use.
Return codes: The amount of return codes you are receiving, indicates that you have some software problems (missing libraries, devices, wrong tooltypes in icons), possibly caused by buggy crossdos moving.
Are you moving the actual .lha archives from your pc to your Amiga and then unpack them on the Amiga, or do you unpack them on the pc and then move the extracted files to the Amiga via 720kB DOS-disks? The latter is known to cause problems.
A very good GUI-based archive unpacking tool is included with OS3.9, or you can also use lha in CLI (text) mode, download the latest from aminet as a self-extracting archive.
-Paul