When you right click on a VM via the vSphere client and click edit setting you will see your hard disk enumerated as follows:

Hard disk 1 - Virtual Disk

Hard disk 2 - Virtual Disk

However CentOS/RHEL enumerates your disks using a different nomenclature. See below:

Disk /dev/sda: 21.4 GB, 21474836480 bytes
255 heads, 63 sectors/track, 2610 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *           1        1044     8385898+  83  Linux
/dev/sda2            1045        2349    10482412+  83  Linux
/dev/sda3            2350        2610     2096482+  82  Linux swap

Disk /dev/sdb: 268.4 GB, 268435456000 bytes
255 heads, 63 sectors/track, 32635 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

   Device Boot      Start         End      Blocks   Id  System
/dev/sdb1               1       32635   262140606   83  Linux

[[truncated]]

While you can assume that the your first disk listed in ESX is your first disk listed via the OS, this is not always the case. You can attempt to compare disk sizes to further determine which disk is which, however the sizes do not always match up.

The best way to identify disks is via the Virtual Device Node, which is visible via the virtual disk properties in ESX and via the command lsscsi in Centos/OS.

More than likely lsscsi is not installed via default so you are going to either need to install via yum, or download a rpm and install. Once installed run as seen below

>lsscsi
[0:0:0:0]    disk    VMware   Virtual disk     1.0   /dev/sda
[0:0:1:0]    disk    VMware   Virtual disk     1.0   /dev/sdb
[0:0:2:0]    disk    VMware   Virtual disk     1.0   /dev/sdc
[0:0:3:0]    disk    VMware   Virtual disk     1.0   /dev/sdd

The first two numbers are unique to the scsi controller and the second two numbers indicate the disk number which can be seen in the disk properties in the vsphere client properties.

For example in vmware Hard disk 4 has a virtual device id of “SCSI (0:3) Hard Disk 4, and by running lsscsi I know for sure that this disk is /dev/sdd. Now I can remove the disk with confidence from esx and have to worry that I removed the wrong disk.