Tidy up old Windows Server Backup snapshots

Question

I'm running wbadmin from a scheduled job, backing up my C: and D: drives to my E: and (I believe!) including the system state:

wbadmin start backup -backuptarget:e: -include:c:,d: -allCritical -noVerify -quiet

I'd like to delete old backups, but I'm concerned that all the information I can find says to use wbadmin to delete old system state backups, and vssadmin to delete other backups. As far as I know, my backups ARE system state backups, but are using VSS on E: for storage, so I'm worried about trying either of these techniques for fear of losing all my backups.

This is a home network, so I don't have a spare server to test this on.

I'm also happy to simply restrict the space used on E:, but I can't make sense of the difference between the /for and /on parameters of the relevant vssadmin command.

For reference, here's the output of vssadmin show shadows:

Contents of shadow copy set ID: {xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx}
   Contained 1 shadow copies at creation time: 07/01/2011 08:12:05
      Shadow Copy ID: {xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx}
         Original Volume: (E:)\\?\Volume{xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx}\
         Shadow Copy Volume: \\?\GLOBALROOT\Device\HarddiskVolumeShadowCopy83
         Originating Machine: x.y.com
         Service Machine: x.y.com
         Provider: 'Microsoft Software Shadow Copy provider 1.0'
         Type: DataVolumeRollback
         Attributes: Persistent, No auto release, No writers, Differential

[... repeated a lot...]

vssadmin show shadowstorage:

Shadow Copy Storage association
   For volume: (C:)\\?\Volume{xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx}\
   Shadow Copy Storage volume: (C:)\\?\Volume{xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx}\
   Used Shadow Copy Storage space: 0 B
   Allocated Shadow Copy Storage space: 0 B
   Maximum Shadow Copy Storage space: 5.859 GB

Shadow Copy Storage association
   For volume: (D:)\\?\Volume{xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx}\
   Shadow Copy Storage volume: (D:)\\?\Volume{xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx}\
   Used Shadow Copy Storage space: 0 B
   Allocated Shadow Copy Storage space: 0 B
   Maximum Shadow Copy Storage space: 40.317 GB

Shadow Copy Storage association
   For volume: (E:)\\?\Volume{xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx}\
   Shadow Copy Storage volume: (E:)\\?\Volume{xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx}\
   Used Shadow Copy Storage space: 168.284 GB
   Allocated Shadow Copy Storage space: 171.15 GB
   Maximum Shadow Copy Storage space: UNBOUNDED

wbadmin get versions:

Backup time: 07/01/2011 03:00
Backup target: 1394/USB Disk labeled xxxxxxxxx(E:)
Version identifier: 01/07/2011-03:00
Can Recover: Volume(s), File(s), Application(s), Bare Metal Recovery, System State

[... repeated a lot...]