An Image for Windows backup using VSS fails when backing up one or more partitions on a drive that includes a partition encrypted with BitLocker. The Windows Event Log shows an event ID 4001 error for VSS. Image for Windows reverts to using PHYLock, which will back up BitLocker partitions in their encrypted state instead of backing up only used space.
Source: VSS, Event ID: 4001
Volume Shadow Copy Service error: Cannot find diff areas for creating shadow copies. Add at least one NTFS drive to the system with enough free space. The free space needed is at least 32 Mb for each volume to be shadow copied.
The partitions being backed up lack sufficient free space to create the shadow copies. Free space on the BitLocker partition(s) is not used for unencrypted partitions.
If VSS backups on the drive were previously successful and now fail with this error, it's most likely something has changed that reduced the free space available for use by VSS. For example, the WinRE.wim file on the Recovery partition is larger than before due to a Windows update/upgrade or updating WinRE to TBWinRE (or IFW WinRE).
This issue can occur when backing up a normal Windows drive (or unencrypted partitions on the drive) where Windows is BitLocker encrypted. For example, if the drive consists of an EFI System partition, a Recovery (WinRE) partition, and the Windows partition (encrypted), the backup will fail if the Recovery partition doesn't have enough free space available.
Sufficient free space must exist on an NTFS partition that is not encrypted (in the example above, the Recovery partition is the only unencrypted NTFS partition). The free space amount specified as needed is at least 32MB per volume. However, the actual amount necessary may be more depending on the system and what's being backed up. Obtaining more free space on the Recovery partition may not be possible (the winre.wim file may almost completely fill it). Instead, it may be necessary to resize the partition larger or create a new empty unencrypted NTFS partition on the drive with an adequate size (100-200MB, for example).
Otherwise, if an entire disk image backup using VSS isn't required, it may be possible to use PHYLock to back up the unencrypted partitions and VSS used to create a separate backup of the BitLocker partition(s).
Checking or Resizing the Shadow Copy Storage Associations
The Windows vssadmin utility can be used to list and resize the Shadow Copy Storage associations. Run the commands from an administrator command prompt.
To list the associations, run the list shadowstorage command with any additional parameters (as determined by your system):
vssadmin list shadowstorage
vssadmin list shadowstorage /On=C:
vssadmin list shadowstorage /For=C:
To resize the associations, run the resize shadowstorage command with the required additional parameters (as determined by your system):
vssadmin resize shadowstorage /For=C: /On=C: /MaxSize=900MB
For more help on vssadmin usage, run the command with the /? parameter. For example:
vssadmin list shadowstorage /?
vssadmin resize shadowstorage /?