There was a VirtualBox VDI file which EnCase and FTK Imager couldn't mount - EnCase v8.05 was unable to even read the partitions while FTK Imager v4.1.1.1 could read the partitions but not the filesystem in the partitions.
Initially I tried to convert to VMDK format using VirtualBox's vboxmanage internalcommands:
vboxmanage internalcommands converthd -srcformat VDI -dstformat VMDK <inputfile> <outputfile>
but it produced the following error:
VBoxManage.exe: error: Cannot copy the image: VERR_VD_INVALID_TYPE
Subsequently, thanks to the post by Namareba食べたい, I successfully converted the image file with the following command:
vboxmanage clonemedium disk <inputfile> <outputfile> --format VMDK
and the resultant VMDK file was read and mounted successfully by EnCase.
