finder

Old imac found a flash drive fine yesterday, but i accidentally closed the drive's finder window and got the warning about ejecting it properly first. But now finder doesn't recognize it. I've made sure external drives is checked. I took the drive out, rebooted and tried again and still no joy. Other flash drives working fine. But now the new 1T one. I'm trying to backup from a mid-2007 imac running el Capitan to move files to my new imac. What do I do now?