I initially only had my 4TB drive formatted GPT, (since I couldn't use it otherwise or only as a 2TB drive if done as MBR). Just after reading your last post I thought I would try and see if I could convert my other drives to GPT. Used MiniTool again and have now converted my 2TB Hitachi and 480GB SanDisk SSD's to GPT. Only took about 10-15 seconds each to convert? Sure enough they are now displayed as GPT;
Didn't want to take a chance on my OS drive just yet. The drive on the bottom of the list is actually a 4TB Seagate NAS drive connected to my router and it appears to use it's own propriety disk controller, (I believe it is so that older OS's [and android] can see and access the 4TB drive). Ran a couple of checks on the disks and can confirm that the data on both drives is fully intact.
With regards to your method of cloning the Z620 drive and putting into the Z420, does the Z420 motherboard share the same chip sets? e.g. SATA/SAS controllers, etc.? Have you done anything similar in the past in this regard? Would be very interested to know if this works. I know you can pull an OS drive from a Z620 and stick it into another Z620 but not to a different model?
When I bought my Z600 it was supplied with the original HP recovery/restore discs (3x DVD's) but I bought the Z620 on ebay and didn't get any DVD's or even a recovery partition so didn't have a set of WIndows 7 Professional disks. Reading on other forums it stated that you needed the full retail version of Windows to create a UEFI boot system. Something about additional files on the retail version compared to OEM versions. However, you are correct in that the UEFI procedure on the HP website does not mention any special OS install disc requirements, i.e. retail or OEM version. Probably could have purchased a much cheaper OEM version of windows or contacted HP for a set, (although my Z620 is out of warranty period).