increasing a VMDK thick provision lazy zeroed

Hi,

I am running ESXI 5.10 Build 1065491 (This is the Free version of ESXI)  VSphere Client 5.10 build 1064113

I have 1 windows server 2008 R2 SP1 with 2 VMDK's Drive c: had 60GB assigned to it but only has `1GB left, when I try if increase it the options are greyed out (please see below),  

Any help would be grateful.

Harvey