Can I use any usb device or MMC for web proxy in my existing mikrotik box that have only one IDE port.

Yes you can use usb devices for stores. On my device it show's as usb1, usb2 etc. in Stores -> Disks.

i can not change cache drive in web proxy setting. why? what is the way using usb drive for cache?

Drives are managed by "stores". To tell web-proxy to store cache on different drive, go to either Stores or System -> Stores in winbox or /stores on console and add a new web-proxy store. If more that one store exists, you can activate the one you want to use.

I have done that. But web proxy do not use the flash drive.

I'm using 3.27 on x86 and it seems to work ok. Try upgrading to latest version.

Make sure you don't have any configuration missing: http://wiki.mikrotik.com/wiki/Proxy_on_ ... rnal_drive

my proxy is already running but no disk cache. so that i want to do cache in flash disk. but one thing is not clear, if i dont use ISP DNS ?

my proxy is already running but no disk cache. so that i want to do cache in flash disk. but one thing is not clear, if i dont use ISP DNS ?

What RouterOS version are you using?

Mikrtotik 3.22 version currently installed.