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I found I can only add three virtual disks to a guest in a VMware Workstation VM. When I tried to add the fourth disk, the IDE option is disabled automatically.

Does anyone know why?

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The limit is actually 4 IDE devices, I am guessing you also have an IDE optical drive on the virtual machine.

As for the reason, this is simply down to the virtual hardware that VMWare Workstation emulates has the standard Primary and Secondary sockets which many motherboards have. Each of sockets supports a Master and Slave device resulting in 4 devices total.

I have tried to do a bit of research, and, I am sure it is technically possible to have more IDE sockets, but, I can't find a single motherboard that shipped with more than 2 - based on this, I am guessing BIOS manufacturers never implemented the ability for more than 2.

IDE also has a lot of restrictions compared to SCSI - so, I am guessing that VMWare added this for backward compatibility reasons only and were never intending people to have many devices.

Here is a picture from the Virtual BIOS (F2 on start).

enter image description here

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But what about PATA controller cards? They would have allowed for more than 4 devices... – AndrejaKo Oct 27 '11 at 10:30
@AndrejaKo I fully agree and thought about this, however, looking around VMware Workstation, I am guessing that they have not made a emulated/virtual controller card, so it is just limited to the 4 that the virtual bios/hardware does support... Where as, they have much higher support for SCSI hardware (Just tested with 30 hard drives and it works!). – William Hilsum Oct 27 '11 at 11:38
This is the right answer. The BIOS for the virtual machine support 2 IDE sockets and there hasn't been enough need for more than that because most users use SCSI emulation for their virtual disks. – kbyrd Oct 27 '11 at 19:33
@kbyrd ... Saw your Linked in address on your profile here... Seeing your job, it is nice to get confirmation from you! – William Hilsum Oct 28 '11 at 9:50

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