You mention Windows 7, but not what type of external disks you are trying to connect.
If they are USB3 then that matters.
Unfortunately Windows 7 itself does not come with USB3 support (this is sometimes referenced as windows 7 not having an inbox driver for USB3).
In order to use a USB3 device under VMware Fusion you have the following options:
- Use an operating system with USB3 support (eg. Windows 8.x or similar)
- Use a USB2 hub or USB2 cable between your computer and the USB3 device you want to use to downgrade the USB3 interface to USB2
- Connect to a USB2 connector on your computer if your computer provides such a connector.
- You can try setting the USB compatibility under menu Virtual Machine -> Settings -> USB & Bluetooth -> Advanced -> Compatibility -> USB2
Starting with VMware Fusion 8 your Windows 7 guests can have USB3 support, but you will have to install an additional driver.
Have a look at the following KB article for the details: http://kb.vmware.com/kb/2128105
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