Windows 7 does not support USB 3 drives. (Windows 8 does support USB 3.0) There is no workaround.
(Installing HFS4Win does not resolve issue).
You need USB 2.0 for all drives using Windows 7 on VM Fusion 5
As per avanish321 you may try an external USB 2 hub but not sure if USB 3 will work through a USB 2 hub (working on it but haven't tried)
Also, remember to use NTFS format for drive (Windows 7) as FAT 32 won't store files over 4 GB.