How many times have you not had sufficient space on your virtual server due to growing exchange data stores, SQL databases, or Data Protection Manager backups? It can be a daunting task, in smaller networks, to have sufficient drive space on VM hosts. Using Microsoft Hyper V server 2008, I've setup test and dev machines and just hosted the VM files off on an external hard drive.
In the past external hard drives running USB 1.1 and 2.0 were too slow to host a file such as a .VHD effectively. Within the last couple of years technologies like e-Sata and USB 3.0 have emerged as great choice's for an external interface. These newer external ports offer superior read and write throughput performance for hard drives. You can purchase expansion cards that give your server the ability to have these ports. In the case of e-Sata, you can also employ Raid setups using external drive housings that support raid 0,1,5,10 and others.
I've found that, depending on your virtualization hosts configuration, you can setup or relocate .VHD files onto an external drive. Also you could add an external drive to house all of your machine backups and images on drives up to 3TB in size. With the performance of both USB 3.0 and e-Sata you can create, deploy, and restore in a fairly quick amount of time. Non-essential or lightly used VM's could be hosted on a single external drive with higher use possible using an e-Sata Raid setup. This solution would be very easy to implement with many server virtualization systems and very inexpensive compared to upgrading the drives in a server.