I tried to extroot my router following this wiki guide: Wiki page Extroot. It didn't work on a D-Link DIR-645, not with 18.6.4 nor 18.6.5. But THIS tutorial did: Okami-Tutorial. Don't have really a clue what might be the reason, maybe the different USB driver kernel modules, that are installed in both tutorials, which seem to be different in sight of USB versions (v1.1 vs. v2.0?), but I suggest to give at least a hint to future readers to minimize the potential frustration level for newbie users.
I always use the Wiki guide and never had a problem with USB 2.0. I hava an external HDD attached to my router as NAS and a flash stick as ExtRoot. You may have missed something that caused it not to work.