Reason:
IOmega eGo 2.5" 500gb hdd (SATA over usb and usb powered) reconnects from time to time without any warning interrupting any operation. Different cables (even double power cables), hubs and ports where tried. No difference. My best guess: lots of bad or slow sectors.

All day today I was getting myself up to speed with software technologies and tools available.
It was not hard to find tools that are capable of running sector diagnostics. (my favorite: HDDScan)
It was hard to find tools that can find and mark slow sectors as bad. I had hopes for Markbad and MHDD. Unfortunately MHDD magic boot disk (MHDD+DOS USB driver) stuck at USB devises detection.

Is there any other software that can mark slow sectors as bad for external hdd?
(side questions: what is a max number of bad sectors that can be marked?)

P.S. for future: what portable hdd would you find most reliable?

It would be great if there was possibility of at least manually set bad sectors. Currently HDDScan is scanning hdd for slow blocks. (pic attached)

As it turns out part of the problem was corrupter NTFS + bad sectors and the other part was Windows 7 "USB selective suspend". Can be found in control panel (pic attached).