Same story for me.
Blinking green light, can't turn on. I used with an external usb 2.5 harddisk.
I read this thread and i searched if i have a 5volts x 2A in my "computer's box"...
I found a power adapter for external HD USB 2.0.
I then try to identify 5v and ground
After that, i cut original wires and put two black wires together and two red wires together.
Now my SheevaPlug is up and running.
P.s. now i migrated my usb 2.5 hd to an usb 3.5 powered hd
P.P.s. I don't know why, but now i got "usb 1-1: reset high speed USB device using ehci_marvell and address 2" errors...