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Question Can't get system to work on GPT so my 3TB HDD's have 2TB storage limit.

Jun 24, 2020
I bought two 3TB Hardrives to replace my 1TB Hardrive. I want to set the two 3TB Hardrives up in RAID 1 as a mirror to have one as a backup in case the other fails. I just wanted to get it working so I decided to deal with this later and just copy my data over. I now realize this was a mistake and I'll have to wipe the hardrives to set them up in RAID.

Right now I have my data cloned from the 1TB drive onto one of the 3TB hardrives without the mirror set up, but it is set up using GPT so it is limited to 2TB of storage and I would like to utilize the other 1TB. However, when I attempt to convert the HDD to GPT to use the 3rd TB my system won't boot and leads me to the windows repair screen. I know my need to set it up using UEFI instead of Legacy and I have attempted to change it in my BIOS but have, apparently, been unsuccessful, seeing as it won't boot.

I decided to give up on this problem and deal with the unallocated 2TB but I have finally decided to set up my drives in RAID 1 by cloning my data back to my 1TB drive, setting it up, and cloning it back. I would like to fix my GPT boot problem while I'm at it. If anyone has any tips on how to get my system to boot using GPT, or a way to set up RAID 1 without wiping the disks, I would appreciate it.

Windows10 64 bit
ASRock B360 Pro4 Motherboard
1TB Seagate Barracuda HDD
2x 3TB Seagate Barracuda HDD