Slow network transfer on gigabit between Win7 and Ubuntu Samba (26 MB/s)

The hard drive almost certainly only supports ~26MBps. Even the latest round of SATA drives can barely pull double that number (under ideal conditions).

That beginning speed of 120MBps isn't even really possible; 1000Mb / 8 * 80% = ~100MBps (realistic max). Windows caching is playing with the numbers, hence the high initial speed and after a while the number settles to the sustainable throughput.