Oh man, I hope it's not the usb port bricking, I got same issue recently; maybe it's software related (latest CSSU stable + KP50 + H.E.N.).

Anyway trying a couple of times I could manage to connect either in mass storage and pc suite mode; after connection I tried to move gently the mini-usb connector attached to N900 to see if the problem was related to loose pins... nothing happened, I was still connected...

I need to say my USB port does not seem loose, it's not "hard soldered" and to preserve it I got 2 batteries and an external charger... I connect the usb cable few times in months, just to tranfer large files (otherwise wlan)...

Next time I'll let you know if the problem still persists...