following happens with both zabbix_get and zabbix_agentd version 1.8.4 and 1.4.4:
zabbix serverside:
/usr/local/bin/zabbix_get -s 10.0.0.231 -k vfs.fs.size[d:,free]
ZBX_NOTSUPPORTED
clientside:
>zabbix_agentd.exe --test vfs.fs.size[d:,free]
vfs.fs.size[d:,free] [u|40308572160]
The C-drive works as it should be. Difference between C-drive and D-drive is that the D-drive is an iscsi target on a Synology NAS. Both drives are formatted NTFS. D-drive is 400someting GB (so not the infamous 2GB+ I read about)
Attachment is a strace made in Linux from the "zabbix_get -s 10.0.0.231 -k vfs.fs.size[d:,free]" command.
If you need more info, let me know.