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Here are some handy script lines for working out what CDs you have connected which can cause Vmotion to fail.
You don’t always want to disconnect all CDs. If you have .ISOs on shared storage Vmotion will be fine as long as the hosts can both see the .ISO files.

Get the names of VMs with connected CD drives:

get-vm | where { $_ | get-cddrive | where { $_.ConnectionState.Connected -eq "true" } } | select Name

Get the names of VMs with connected .ISOs

get-vm | where { $_ | get-cddrive | where { $_.ConnectionState.Connected -eq "true" -and $_.ISOPath -like "*.ISO*"} } | select Name, @{Name=".ISO Path";Expression={(Get-CDDrive $_).isopath }}

Disconnect all VMs where the CD Drive is connected and it is not an .ISO

$VMs = Get-VM
$CDConnected = Get-CDDrive $VMs | where { ($_.ConnectionState.Connected -eq "true") -and ($_.ISOPath -notlike "*.ISO*")}
If ($CDConnected -ne $null) {Set-CDDrive -connected 0 -StartConnected 0 $CDConnected -Confirm:$false }

or another slightly quicker and dirtier way thanks to commenter José de Alonso

Get-VM | Get-CDDrive | Where {$_.ISOPath -ne $null} | Set-CDDrive -NoMedia -Confirm:$false