WL-500gP problem, strange behaviour

I have WL-500gP with Oleg's firmware, currently 1.9.2.7-10. I was playing with removable HDD (Welland ME-740PS) for two days. Apart from problem I have with HDD with the power saving, something strange start to happen:

When I log in by putty telnet into routers console, I often have problems with typing commands, characters misses sometimes. Additionally every 15 seconds a block (very small chessboard) character is displayed on the screen, which is coming on the command input. I mean, when I type enter after couple of such characters appearing, I got a message about wrong command.

Is it something wrong with the hardware?