help with usb problim

hi >> i am use rb 2011 . but my problem is on more rb model >>
when i change user-manager db path to usb and after 50 active user the user-manager accouting have a problem >>
( RADIUS accounting request not sent: no response )
i try make timeout 3000 and 9999 but the problem is still >
when i take off the accounting option from hotspot server profile the problem is go >>
and when i use user-manager db path on internal memory the problem is go >
the problem is from usb db i think ther is a slow rate>
can any one help me please >>
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RB2011 has RB license level 5 with a limit of 50 active User Manager sessions.
https://wiki.mikrotik.com/wiki/Manual:License

thanks >> but i have same problem with rb1100x2 level 6 license >> bro the problem is when i chang the user-man databaes path to usb … this is the point