It’s in the FBI’s possession, but the FBI had never actually admitted that to the public previously. They have been claiming that all the emails Hillary’s lawyers deleted are “missing”, when they had the backup device sitting in their possession all the time.
I wonder if Hillary's lawyers initially denied they had the small storage devices in their possession (e.g. thumb drives and Seagate exterior hard drives) - then handed them over in return for being cut a break by the FBI?
Someday Hillary's deleted emails will be divulged. Preferably they will be made public by our own government (rather than by the Russians, Chinese, Iranians etc - who no doubt already have them).