I happened to convert a virtual machine from HyperV to vSphere through vmware converter but at the end of the conversion the vm disk (it had a single disk) was created as Eide (so it was left unchanged), consequently the vm wasn't able to boot properly. The same thing can happens when you convert an old physical machine to virtual and you don't select the correct adapter during the wizard.
In cases like this you can convert a disk from Eide to scsi going to directly edit the file vmdk. Here you can find the official vmware kb “Converting a virtual IDE disk to a virtual SCSI disk (1016192)”: