PC Only Boots When the Old HDD Is Connected After Installing a New SATA SSD

Make sure you are selecting the correct boot entry in the firmware settings. Under UEFI, choose “Windows Boot Manager” associated with the M.2 drive rather than just the SATA SSD. Also confirm the new SSD is detected there. If it is detected but no boot entry works without the HDD, repair the boot files while the HDD is disconnected.

Check the old HDD in Disk Management while it is connected. It may contain the EFI/System Reserved partition that your Windows installation on the M.2 drive actually uses to start. This can happen when Windows was installed while multiple drives were connected. The cleanest fix is usually to disconnect every other storage device, boot from a Windows installer, and repair or reinstall Windows with only the intended system drive connected.

The new SATA SSD does not need to be bootable if Windows is installed on the M.2 drive. Set the M.2 drive or its Windows Boot Manager entry as the first boot option, then check whether the new SSD is detected in firmware. Once Windows starts, initialize and format the new SSD from Disk Management. If the computer only starts when the HDD is attached, its EFI partition is probably being used.

Simply copying files, or using a basic disk-mirroring tool, may not create a bootable Windows installation. If you intended to move Windows to the new SSD, use cloning software that supports Windows boot partitions, or perform a fresh installation. For a fresh install, unplug the M.2 drive and all other storage devices first so the installer places the EFI and recovery partitions on the correct drive.