Windows 10 Doesn't Always Recognize my GPT drive as a GPT

[Tremelo]

Almost a year ago I bought a 4TB hard drive and used the Mini Tool Partition Wizard to split it into three partitions: a 2TB main 'C: drive' boot drive partition and two 1TB data drive partitions. Things worked fine until about a month ago when I noticed the data drive partitions were not always showing up in Windows Explorer. It was random. Sometimes I’d boot up and all three drive partitions would show up in Windows Explorer, sometimes only the main drive partition (the ‘C:’ drive) would.

Last night when only the boot drive partition was showing up I looked at Disk Management and could see all 4TB of the hard drive but the last 2TB unallocated. I opened the Mini Tool Partition Wizard and and saw the same thing. When I tried to see if I could make a partition out of the unallocated 2TB I got an error message 'This disk uses MBR format, the disk space beyond 2TB will be unusable.' I didn’t change anything and rebooted the PC. BOOM! The other two drive partitions appeared again with all their data! But then I rebooted a second time (again without making changes) and I was back to just the boot C: drive partition showing up.

Does anyone have any idea why Windows 10 apparently sometimes sees this 4TB drive as a GPT drive with three partitions and sometimes with an MBR drive with only one?

Some notes that I’m sure will be asked for:

My OS: Windows 10 Pro 64 bit
My motherboard: Gigabyte 990FXA-UD3 (rrev 3)
The hard drive: ST4000DX001-1CE168
OS Loads: Legacy BIOS, but I'm pretty sure my OS will handle UEFI and I'm willing to convert to UEFI if it will solf my problems.

PS - I’m not sure if this is related or not, but during the same time period sometimes when I’d boot up the motherboard wouldn’t find any OS to read and I’d get an error message about no OS found. I chalked this up to a loose SATA cable and have not had this error since I replaced the cable *knock on wood*. Though, as I type this though I think ‘Hrm, maybe it IS related?’

 

[Colif]

Windows should not be able to do that... The way a GPT drive boots is different than a MBR system, the drive can only be formatted as GPT or MBR, not both... so this is bizarre unless for some reason drive has MBR & GPT boot files, it should reject one of them at startup

Can you show us a screenshot of disk management?

Patented DualBIOS with Hybrid EFI technology for 3TB HDD support

http://www.gigabyte.com.au/Motherboard/GA-990FXA-UD3-rev-10#ov

Only mentions 3tb as I would assume there wasn't anything bigger at time.

if you look on the BIOS features tab of BIOS, under Boot mode selection, its likely you have UEFI & Legacy set. You can change this to UEFI only and it should always show all the drive - I don't understand why PC would boot as legacy though unless you have 2 drives in PC?

 

[Tremelo]

I'll work on the screen shots. Currently it has booted up in the C: drive only mode so it is showing 'MBR'. I'm assuming the next time I get the additional partitions it will show GPT

I DO have two other physical data drives in the PC, one of which used to be the main drive.

And I'm not set up for UEFI yet. I am pretty sure the hardware will support it but I haven't gone through the steps yet to convert over concerns that if I convert when Windows sees the 4TB drive as a MBR drive then the data on the other GPT partitions might be nuked.

 

[Colif]

convert how? As far as I can tell your drive is GPT now as you wouldn't have the last 2 partitions if it was set up as MBR. MBR can only use 2tb so if you had formatted drive as MBR you wouldn't have those other 2 partitions.

I know whats happening (I think), is Windows 10 still on the old drive? What happens is 1 time you boot up, it uses UEFI and boots off the 4tb drive which is formatted as GPT, but the other time it uses the old hdd to boot off which is set up as MBR and cannot see the other 2 partitions. Even if windows isn't on old drive, win 10 may have added itself to an old MBR on the hdd and it might boot up using that when Bios flips to legacy.

What happens if you take old drive out and boot up? could just remove old drive from boot order as well. that might work just as easily.

 

[Tremelo]

I know whats happening (I think), is Windows 10 still on the old drive? What happens is 1 time you boot up, it uses UEFI and boots off the 4tb drive which is formatted as GPT, but the other time it uses the old hdd to boot off which is set up as MBR and cannot see the other 2 partitions. Even if windows isn't on old drive, win 10 may have added itself to an old MBR on the hdd and it might boot up using that when Bios flips to legacy.

What happens if you take old drive out and boot up? could just remove old drive from boot order as well. that might work just as easily.

You may be on to something? The old drive does not have Windows 10 on it anymore, no, but it was a MBR drive. I went back into the bios and switched a few things around. Most notably I made sure the boot order was correct. For some reason the 4TB drive was not at the top of the list and was behind the old drive. I bumped the 4TB drive up to the top and made sure to allow for UEFI and Legacy loading (even though I have not set up UEFI yet). I am now seeing all partitions. But the drive looks different in Disk Manager than it did last night. Now it appears as two separate drives, one 2048GB drive, one 1678GB drive. I'm not complaining mind you. If this works consistently then I'm fine! I'm not quite ready to mark it as 'Solved' just yet though.

https://www.dropbox.com/s/7iggf6nv3ufgfzr/diskmanager.png?dl=0

Thanks for your help so far! The thought that the 'old' main drive might be tripping up Windows into thinking the new main drive is MBR may (hopefully!) be what fixes this!

 

[Colif]

You don't have to set up UEFI, its already there. You wouldn't have the choice of legacy/UEFI if bios was just legacy.

I don't understand why you have the drive showing as 2 discs there, it shouldn't be. Not sure why your boot partition is 1gb, it doesn't need that much normally. think i need a 2nd opinion on whats going on. Strange C drive isn't disc 0 but I will let that go.