USB drive keeps disconnecting and reconnecting

It's over two years later but I figured I may as well answer this question just in case anyone else ever has the same issue.

It turns out that one of the USB2 ports on this old MacBook Pro was not working properly. Not sure exactly what the issue was, but I ended up buying a USB3 expansion card to get better speed out of the drives, and after using that both drives performed perfectly. This is the first and only time I've ever had a USB port actually be the cause of a problem like this.

Maybe the replacement drive isn't exactly identical and requires a bit more power. Disconnecting and reconnecting is typical for underpowered drives. Try to connect external power source if the disk's enclosure supports it.

This "Seagate Backup Plus" is an external disk enclosure, right? The disk might be OK but maybe it's the enclosure's fault? Or the USB cable is bad. If you haven't changed anthing else (kernel upgrade?) since the older disk died, I'd suspect a hardware problem. Without attempting to import the disk to the ZFS pool, can you dd from the disk from start to end w/o errors?

I had a similar problem.

I went for a hard solution: getting an internal SATA drive and no longer be at the mercy of USB issues.

Perhaps this is also cost effective for you, your time is valuable.