I also have and HDD which works surprisingly good considering it's 5400RPM.
To answer your questions:
a) 3.13.0-52-lowlatency
b) As little as 2 files totalling 2,2 GiB, the first is written very fast and
when in starts with the other there is a delay accompanied by intermittent
system freezes until the operation is over.
c) I'm copying from HDD to SSD (/home and also root).
d) There aren't many cables to my disposal from what I'm aware.
e) The system is a HP ProBook 430 G2 with the SSD on the M2 slot (Adata SSD
Premier Pro SP900 128GB M.2).
After I enabled discard in crypttab and grub I see slight improvement but the
problem persist.
Also, I noticed similar behaviour when writing to usb stick (first file is
written fast and at the second is a short delay), but there is no system
freeze because the OS is not there probably.
