according to "free", about 35% are used for buffers/cache, and the rest is stated as free. OK. So that seems to be normal.
Yes, I was using CIFS only, but with 5 different machines without any virus scan tool and the firewall was even disabled, also tested through direct connection.
Yes, the problem is still there after reboot. Jumbo Frames are not in use, because they cause lagging from time to time (switches are GS108v2 and v3)
Maybe it's important that my files are 20-80GB each?
I now used http-Protocol and got much better speed: about 108MB/s damn. thats a difference!
EDIT: The file was uploaded, but didn't appear yet. Did I skip an essential step?
EDIT2: Tried with ftp: 61MB/s average, file appears on ReadyNAS-Share.
Addons: Only SSH-acces, the readyNAS has a clean factory default with T42.