i can't see mi flash drive

Before suggestions can be made, it would help to know which model iMac that you have and what OS version it's running.

Mac OS version?

Help! ( covers a lot of issues. )

Is this a new flash drive?

Did it work before?

In another comptuer?

Have you tried a different port?

Have you looked in

Macintosh-HD -> Applications -> Utilities ->Disk Utility

for the drive?