Well, from the ramblings of other blogs and the community forums, that ^ error string from the vSphere Client could mean just about anything could be wrong with your storage, your LUNs or your storage network.
In my case, after a long call with Dell EqualLogic and VMware, it turned out to be MTU size. In order to make my world right again, the MTU size on the Dell 6224 switches needed to be higher than the MTU size on the surrounding infrastructure. So: vSwitch, VMKernel and EqualLogic mtu: 9000. 6224's: 9126
Previous to that, there were logins and logouts on the iSCSI side, all sorts of errors seen in the vmkernel logs, but nothing...absolutely nothing...pointed in the direction of the MTU.
So, my short story that was actually really long is to show you, dear reader, that the first blog post that you find with this error message might not be your problem, it might not be the third or fifth solution you find. You might need to get help from someone else to check your configuration for something stupid that you've missed because you are less familiar with jumbo frames.