What might cause the upgrade to be unsuccessful?

B is NOT the answer to this question!

http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=2052329

All HW specs are OK.
ESXi 5.5 System Requirements

When installing or upgrading to ESXi 5.5, ensure that the host meets these minimum hardware configurations supported by ESXi 5.5:
Your hardware is compliant on the VMware Compatibility Guide. This includes:

System compatibility
I/O compatibility (Network and HBA cards)
Storage compatibility
Backup software compatibility

You have a 64-bit processor. VMware ESXi 5.5 only installs and runs on servers with 64-bit x86 CPUs. It also only supports LAHF and SAHF CPU instructions.
You have an ESXi 5.5 host machine with at least two cores.
The NX/XD bit is enabled for the CPU in the BIOS.
Your processor is supported. ESXi supports a broad range of x64 multicore processors. For a complete list of supported processors, see the VMware Compatibility Guide.
You have 4GB RAM. This is the minimum required to install ESXi 5.5. Provide at least 8GB of RAM to take full advantage of ESXi features and run virtual machines in typical production environments.
Support for hardware virtualization (Intel VT-x or AMD RVI) is enabled on x64 CPUs (to support 64-bit virtual machines). For a complete list of operating systems supported with ESXi, see the VMware Compatibility Guide. Hosts running virtual machines with 64-bit guest operating systems have these hardware requirements:

For AMD Opteron-based systems, the processors must be Opteron Rev E or later.
For Intel Xeon-based systems, the processors must include support for Intel Virtualization Technology (VT). Many servers that include CPUs with VT support might have VT disabled by default, so you must enable VT manually. If your CPUs support VT , but you do not see this option in the BIOS, contact your vendor to request a BIOS version that lets you enable VT support.

Note: To determine whether your server has 64-bit VMware support, download the CPU Identification Utility from the VMware Website.

You have one or more Gigabit or 10GB Ethernet controllers. For a list of supported network adapter models, see the VMware Compatibility Guide.
You have Storage controllers with any combination of one or more of:

Basic SCSI controllers. Adaptec Ultra-160 or Ultra-320, LSI Logic Fusion-MPT, or most NCR/Symbios SCSI.
RAID controllers. Dell PERC (Adaptec RAID or LSI MegaRAID), HP Smart Array RAID, or IBM (Adaptec) ServeRAID controllers.

You have SCSI disk or a local, non-network, RAID LUN with unpartitioned space for the virtual machines.
For Serial ATA (SATA), a disk connected through supported SAS controllers or supported on-board SATA controllers. SATA disks are considered to be remote, not local. These disks are not used as a scratch partition by default because they are seen as remote.

Note: You cannot connect a SATA CD-ROM device to a virtual machine on an ESXi 5.5 host. To use the SATA CD-ROM device, you must use IDE emulation mode.

You are using a supported storage system. ESXi 5.5 supports installing on and booting from these storage systems:

SATA disk drives. SATA disk drives connected behind supported SAS controllers or supported on-board SATA controllers.
LSI1068E (LSISAS3442E)
LSI1068 (SAS 5)
IBM ServeRAID 8K SAS controller
Smart Array P400/256 controller
Dell PERC 5.0.1 controller

SATA disk drives. Supported on-board SATA include:
Intel ICH9
NVIDIA MCP55
ServerWorks HT1000

Note: ESXi does not support using local, internal SATA drives on the host server to create VMFS datastores that are shared across multiple ESXi hosts.

You have Serial Attached SCSI (SAS) disk drives supported for installing ESXi 5.5 and for storing virtual machines on VMFS partitions.
You have dedicated SAN disk on Fibre Channel or iSCSI.
You have USB devices that are supported for installing ESXi .
You can install and boot ESXi from an FCoE LUN using VMware software FCoE adapters and network adapters with FCoE offload capabilities. See the vSphere Storage documentation for information about installing and booting ESXi with software FCoE.