AOFO: Initialization failure V-79-10000-11217 - VSS Snapshot error

Hi

Im new to this please bear with me. I get the folowing error on a lot of my backups using 2010 R2

Backup-
- AOFO: Initialization failure on: "\\SERVER\F:". Advanced Open File Option used: Microsoft Volume Shadow Copy Service (VSS).

V-79-10000-11217 - VSS Snapshot error. The Microsoft Volume Shadow Copy Service (VSS) snapshot failed. Make sure that all provider services and tasks are running. Check the Windows Event Viewer for details.
The event log shows
Snapshot creation failed with error 0x80004005 (Volume SERVER-F: Not enough snapshot reserve to create this snapshot) for target \\.\PHYSICALDRIVE2 (0-8a0906-141d2b707-987000000114cc82).

can I just untick the use AOFO on the backup job and it will skip open files?
OR
can I create a new volume for AOFO to use for each backup?

This particular job has 29GB free on a 250GB drive if I need to create a drive for AOFO what sort of size am I looking at for 250GB?

Thanks