A couple of days ago, I spent two hours setting up two Windows Server 2012 VMs on my ESXi 5.1 cluster and tried to get some performance tests done. When copying multiple ISOs across the network between those two VMs, I received an error that neither of my 5 ISOs could be opened on the destination.

After checking the settings of my VMs, I saw that I used the default e1000e vNICs. Apparently, this is a known issue with Windows Server 2012 VMs using e1000e vNICs, running on top of VMware ESXi 5.0 and 5.1.

The scary part is, the e1000e vNIC is the default vNIC by VMware when creating a new VM. This means, if you don’t carefully select the correct vNIC  type when creating your VM, you could potentially run into the data corruption issue.
The easiest workaround is to change the vNIC type from e1000e to e1000 or VMXNET3. However, if you use DHCP, your VM will get a new IP assigned as the DHCP server will recognize the new NIC.

If you prefer not to change the vNIC type, you might just want to disable TCP segmentation offload on the Windows 2012 VMs.
There are three settings which should be changed:

IPv4 Checksum Offload

 

Large Send Offload (IPv4)

 

TCP Checksum Offload (IPv4)

 

Further details can be found in VMware KBA 2058692.