My older custom-built PC stopped working, so I replaced the motherboard, CPU, and RAM. After reconnecting the existing storage, the system no longer boots from any of the drives, although I can start Ubuntu LTS from a USB drive and mount the disks normally. I have two 256GB Samsung SSDs and a 1TB Western Digital hard drive. The computer previously had a Windows 10 and Ubuntu dual-boot setup, and I can still access the files.
Since each drive has unused space, would it be reasonably safe to create a new partition and install the latest Ubuntu without affecting the existing data? Or should I copy anything important elsewhere, completely wipe one SSD, and perform a clean installation on that disk?
3 Answers
A new partition is possible, but make a backup before changing the partition layout. The safest and simplest approach is to copy your important files off one drive, verify the backup, then use that empty drive for a clean Ubuntu installation.
Before giving up on the old installation, check the firmware settings on the replacement motherboard. The storage controller mode and boot mode—such as IDE, AHCI, RAID, or UEFI versus legacy—may need to match the configuration used when the drives were installed. Even so, a clean install on a backed-up or dedicated drive is the least risky long-term option.
Use the live USB to mount the drives and copy anything valuable to another disk or a reliable backup service first. Once the backup has been checked, wiping one SSD and installing Ubuntu there avoids accidentally damaging the old partitions or boot setup.

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