I must have mixed up the ports and console redirection. From what I can remember I only did what you described above. It would still go to the =trigger device plug event= but this time it didn't lock up and a few seconds later the diskstation login prompt appeared. I will check again when I get back home and update this if I find anything different.
I just got back and re-enabled the serial ports, the default port redirection settings, and it still worked. Whatever it was, it had nothing to do with the serial ports.
After restoring all the bios to UEFI defaults, I am again able to reproduce the problem: last line showing on terminal is =trigger device plug event=. Then it locks up. I then went through all the bios settings that I changed before. The one that made is:
Intel RC Setup -> South Bridge Chipset Configuration -> SATA Configuration -> SATA Mode Selection = IDE
This means you won't be able to use the 4 SATA II ports, but you can use the 8 SATA III ports. That's the answer for Asrock C2550D4I/C2750D4I users.
Sorry for hijacking your thread, thanks heaps to 68rustang for pointing me in the right direction, without which I wouldn't even have thought to go around mucking around with the bios settings.