I installed routerOS to my flash disk but I have problem . it automatically delete’s username and passwords. How can I fix it . . Or which flash disk is best for mikrotik.
Please Helpp . .! 
I doubt flash could erase any username/password.
Check your motherboard, flash the latest BIOS.
Try CF in another motherboard.
Thanks for Reply .. I use IBM x3550 for server. And When I changed flash disk , everything works fine. After 2 week’s I have same prob. it is routeros 5.4 on x86.
Have you tried the same disk in other motherboard?
Perhaps motherboard sends some non-standard commands.
losing usernames and passwords happens when the flash is corrupt - im guessing the system tries to write to it and can’t. sometimes a reboot will read them back in, sometimes not. time to replace that flash.
Thanks For Reply..
yes change ip, I agree with you..But it is second time that I change my flash disk. And I am searching for permanent solution. Do you have any idea !?
buy industrial flash with lifetime warranty. cheap flash from corner store will wear out within a year sometimes.
are you doing a lot of logging to disk? Determine what all the writes are from and tune it. I found that the dhcp server leases at 5 mins and the graphing options were killing mine.