Start Up Issue

A brief background:
The PC would randomly cut out and restart. No BSOD symptoms or warning siren (usually occurs with BSOD). This went on intermittently for a while. Last night, every 10 minutes the computer would restart randomly. On some occasions it would hang in the BIOS. Bare in mind, the CPU and RAM are compatible and although set on overclock, they have been stable for several months.
Other info:
I presumed the HDD was failing, it had done many years of work. I purchased a new SSD and SATA today. Installed both into the tower and took out the previous one.
After successfully installing Win 10 to SSD (Sata for storage), I restart the system to validate the program installs I had added after OS install.
Then the problem hit me again, now this is where I am confused. Why did the system successfully reboot without fault while installing?
After pressing my restart button on my tower, it finally leaves the BIOS splash screen to a “Disk Boot Failure, Insert System Disk and Press Enter” message, so I did the following:
Reset CMOS to Fail-Safe, which ran a memory test. Ensured the SATA HDD which was on SATA1 cable, was assigned as 2nd priority HD. Ensured the SSD, now my C: drive, was set as primary HD. Ensured that boot order was 1) HD, 2) CD 3) USB. Saved all to CMOS. Swapped the order of my RAM around.
All things I have done to no avail. The message still shows
Any help would be much appreciated

Update: I checked my SATA cables once more, with the intentions of changing the order. It appears my CDDrive cable wasn’t probably fitting into the SATA connector on the MB. Instant boot, no issues. I shall update if the problem persists. I will leave this here for any other victims of this issue.
Answer #1
Oh the things that frustrate us! I had a similar problem a year or so back with rebooting and all that sort of stuff. So like you I did the usual and checked all the connections inside the box. Everything went fine for a few weeks then it started giving the same problems again. This time I took all the cables out to have a good look at them and on one of the sata cables (the end that went into one of my HDs) I found a tiny barely visible crack in the bit that pushes into the HD. I replaced the cable and everything has been fine since. I did notice the cable was slightly loose so I assume the cracked connector bit was gradually moving over time due to the vibration of my PC box ……. Annoying isn’t it, especially when you spend all that time going through all the normal routines then end up with the problem still there.
Answer #2
Indeed. It is making me believe the old HD may still be working. Which gives me back my original 500GB
I just need to find a way to connect. My MB only has 3 SATA ports and I’m using two for the SSD and new 1TB HD. The final one, for my disk drive.
Answer #3
get a USB enclosure for it
Answer #4
get a USB enclosure for it
Like a dock?
Answer #5
yup