Need Backup Strategy, Win2k8R2 Hyper-V Host and its 4 VMs.

Nice new big Dell T610, loaded, 4CPUs, 24GB Ram.  Internal Raid 5, total HD space 1.2 TB.
   4 VMs, 250 GB each, all running Win2k8R2 64-bit.

We need to adobt a backup strategy.  We're a small business, spent most of budget on the big box itself.  

What we've looked at so far:

1) "Windows Server Backup",  from the host, Full,  Daily, 4am.  VSS registry tweaks completed.   The target hardware is a simple dedicated external 2TB USB 2.0 Western Digital.    Problem:  Day 1 took nearly 9 hours to complete the backup.  Day 2 took 7 hours.  We can't subject the server to this everyday.   Are the long hours due to the relatively slow USB external HD or maybe because WinSerBk is doing a full backup of entire VM files everytime?

I was hoping that day 2 would be faster, maybe some incremental backup occurring, but now I'm thinking that the incremental backups are not possible with the host's VM files since as a file they change constantly and VSS probably timestamps file files too.  I guess copying 1TB+ everynight would take some time.    

I thought about only doing the WinSerBk every few nights instead of every night, but the WinSerBk settings only seem to allow for Daily at the longest, not every few days or eveen once a week.


2)  MS Data Protection Manager.  We have not actually tried this yet.  We've been told that this requres a pretty heftly hardware investment for server PC and at least the same amt of disk space 1.2TB, ie like a mirrored server same as our one $$ big one.


Any ideas and hints would be appreciated.   500 points.   Thanks.