USB problems

I have a mid 2012 Mac Mini Server connected to a Thunderbolt external HD and a backup RAID via USB. The USB recently started "kicking" the peripheral out with an error message "Drive was ejected improperly"; this was followed by a damaged drive in the RAID. While the drive is being replaced I connected a new 4TB peripheral via USB for TimeMachine backup and the same thing has been happening, usually after about an hour. I also noticed that the keyboard and mouse (also connected via USB through the keyboard) were not responsive. Restart fixed problem, but it keeps coming back. Is this a hardware problem, software or peripheral (doesn't seem likely). Through it all, the Thunderbolt external remains mounted. Help.