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MBR to GPT, now getting “error no such partition” on boot (uninstalled Linux months ago)

Previously this was a dual boot system with Ubuntu and Windows 7. Disk was MBR

I removed Ubuntu. I can’t remember how, but on boot it no-longer asked which OS I wanted, it would just boot straight to Windows 7. Seemed fine, worked for months.

Recently bought a 4TB SSD and cloned my disk to this. Windows 7 booted up fine.

However, I realised it needs to be GPT. I converted the cloned 4TB from MBR to GPT using AOMEI Partition Assistant Pro.

But now when I boot I get:

error: no such partition
Entering rescue mode...
grub rescue>

Obviously Grub didn’t completely disappear!

I thought it might be due to UEFI, so I changed those in the BIOS but I just got a slightly different error.

What’s the best solution to sort this? I do still have the original drive the clone was derived from.

I’d like to keep my OS rather than fresh install.