You should have done that from the beginning!
Well, I use userman since last year and my DB is about 10 MB. At this site there is a maximum of about 50 active users at any time, so I thought that the flash should handle it. And I had tried previously with a SanDisk SDDD3-032G-G46 Ultra 32GB Dual USB stick, but this was remounted with a different name after a reboot or other failure so I abandoned the idea of userman on usb stick. But at least the Intel 80GB SSD has been remounted after reboots with the same name, so for now the system is running.
Now my question is, will the router stay stable in this configuration (with USB-SSD) or will the "disk I/O error" come back? Should I replace the router or wait?