VM Clone Hangs at 99% — Likely a Storage Driver Issue

You're cloning a VM in vCenter — maybe from a template or a thick-provisioned disk to thin — and the progress bar hits 99%, then sits there. Minutes pass, then hours. The task never fails, it just stays at 99%. You check the datastore and the clone's disk is fully created, but the VM won't power on cleanly. The clone's console shows a blank screen or a boot loader error.

What's actually happening here is the clone operation finishes the disk copy, but the post-copy driver configuration step hangs. vCenter tries to reconfigure the SCSI controller from the source's settings to the target's required settings, and the guest OS doesn't respond. The clone completes at the storage level, but the guest can't boot because the SCSI driver is wrong for the target controller type.

This happens most often when you clone a VM that was originally created with a LSI Logic SAS controller to an ESXi host that expects VMware Paravirtual (PVSCSI), or vice versa. It also happens when you clone from a VM with an IDE disk to one using SCSI. The clone task waits for the guest OS to acknowledge the new driver, which it never does.

Root Cause

The root cause is a mismatch between the virtual SCSI controller type in the source VM's hardware profile and the controller type the cloned VM's guest OS has drivers for. vCenter's clone wizard doesn't change the controller type — it copies the source's settings. But if you're cloning to a different host or cluster, the target's VM hardware version might not support the source's controller. vCenter tries to re-enumerate the controller during the clone's final phase, and the guest OS hangs because it doesn't have the right driver loaded.

The real fix is to make sure the guest OS has the correct drivers before you clone, and to explicitly set the controller type in the clone's hardware profile to match what the target host supports.

The Fix

1. Identify the source VM's controller type. Right-click the VM → Edit Settings → expand the SCSI controller. Note if it's LSI Logic SAS, LSI Logic Parallel, VMware Paravirtual, or BusLogic.
2. Check the target ESXi host's supported controller types. In vCenter, select the host, go to the Hardware tab, and look at the Storage section. PVSCSI is recommended for most modern workloads. LSI Logic SAS is common on older hosts.
3. Install the appropriate drivers in the source VM.
   • For Linux: Run lsmod | grep vmw_pvscsi to check for PVSCSI driver. If missing, install vmware-tools or the open-vm-tools package: apt-get install open-vm-tools or yum install open-vm-tools.
   • For Windows: Open Device Manager, expand Storage controllers. If you see 'VMware Paravirtual SCSI controller', you're good. If not, download and install the VMware Tools ISO from vCenter (Actions → Guest OS → Install VMware Tools).
4. Clone the VM again, but this time manually set the controller type. In the clone wizard, go to 'Customize hardware' → expand the SCSI controller → change the 'SCSI Bus Sharing' to 'None' and set the 'SCSI Controller' type to match what the target host supports. If you're unsure, pick VMware Paravirtual — it's the most performant and widely supported on modern ESXi 7.x and 8.x.
5. After cloning, power on the clone. If you get a blue screen or kernel panic, you didn't install the drivers beforehand. Shut down the clone, change the controller back to the source's type, boot it, install the drivers, then shut down and change it to the desired type.

If It Still Fails

If the clone still hangs after the above steps, check two things:

• The target datastore. A failing or oversubscribed datastore can cause the clone to hang at 99% while storage reclamation lags. Check the datastore's latency in vCenter → Monitor → Performance. If latency is over 10ms consistently, migrate the source VM to another datastore first.
• VMware Tools version. On Windows VMs, an outdated VMware Tools (pre-10.0) will cause the driver configuration step to fail silently. Update tools to the latest version on the source before cloning.

If you skip step 3 (installing drivers), you will see the same hang every single time. There is no magic workaround. The guest OS must have the correct driver on disk before the clone reconfigures the controller.