ReadyNAS Ultra 4 Plus Jumbo Frame Issue

Also have you checked to see if you have the latest drivers installed for your NIC?

As I said, the first step is to figure out the actual MTU size of the path. The ping procedure above will do this (also it works with jumbo frames on). As far as expectations go, jumbo frames do not always increase performance. And since they never were standardized (due to technical issues), there are some mixes of equipment where they simply won't work. If your PC NIC has 9014 hard-coded as its only JF size, then it is a poor implementation. Of course the PC can receive 9000 byte frames, so reads should work. When writing, I think both the switch and the ReadyNAS will have to either drop or fragment the JF packet. Maybe you can post both read and write speeds. BTW, it is hard to give much advice w/o knowing more details about your switch and PC setup. Also it would be helpful to know the ultra's network stats when you have jumbo frames on.