The delta disk will be relying on a base disk in order to be a complete picture. If you don't have the associated base file then blocks will be missing that the delta will be expecting. It's possible you could run a disk recovery utility within the guest OS and retrieve some files but really need the base to be sure.
A delta disk with a broken snapshot chain cannot be mounted. Anyway, since the delta file's size is close to the mentioned 500GB you may be able to create a new base disk, modify the required settings to fix the snapshot chain, and access at least some of the data.
Please attach the existing .vmdk descriptor file (SapProd_1-000001.vmdk) to a reply post. Also check whether there are still vmware*.log files in the VM's base folder which contain references to the missing flat file, and attach these .log files too.
If you want to repair this by yourself, the basic steps are:
- create a new virtual disk with the exact same size and type as the missing disk (flat and delta need to have the same number of blocks - see .vmdk descriptor file)
- format the new virtual disk using the same OS/settings as used for the original/missing disk
- fix the snapshot chain by editing the existing snapshot's .vmdk descriptor file (parentFileNameHint = The base disk's .vmdk descriptor file name, and parentCID = the base disk's CID)
- attach the snapshot disk (using the .vmdk descriptor file name) to a virtual machine (Note #1), and reload the VM configuration to make ESXi aware of the changes (see http://kb.vmware.com/kb/1026043)
- create a new snapshot to avoid changes to the current delta file!
This should allow to access the disk. However, due to the missing/recreated flat file, the virtual disk will most likely have a corrupted file system.
Note #1: You cannot use the GUI to attach a snapshot disk to a VM. So what you want to do is to modify/edit the .vmx file and edit the file name for the newly created virtual disk so that it points to the delta disk's descriptor .vmdk name.
PS: Before doing something, take a look at the existing descriptor .vmdk file to ensure that the parent/flat file doesn't exist in anoter folder, i.e. actually still exists.