I have no clue about the iRiver, so I'm just idly speculating, probably uselessly. I just remember that not all ssds would work with ipod conversions.
What happens if you boot it up while it's connected to a source of power other than the PC?
Just wondering whether it's anything to do with the power the ssd requires at start-up, so it can get going when connected to usb simply because it can draw more power from it?
Seems as if the original 1.8" hard drive draws significantly less power than that ssd, according to the figures I've seen.
Can't find anything on whether ssds draw additional power on starting up. There's no spin-up I guess, but I bet there's still some extra power draw for something?
As I say, just speculating, might be barking up the wrong tree, but maybe the ssd can't get enough power from the battery to get going?