Problems keeping USB Drives mounted

I've got a RN516, Firmware 6.18 and I have ongoing problems keeping the USB drives mounted for backups. I have two USB 3.0 drives (one Seagate and one WD). Sometimes they stay mounted for a day and sometimes less. The symptoms are that backups start to fail with "path doesn't exist" and when I try an 'ls' from a shell either I get an empty directory listing or an "Input/output error." A reboot solves the problem. This is my fourth ReadyNAS and the most problematic one by far.