Hi gang You guys seem to know a lot about these things so I hope someone answers!
I had a hard disk fail on me (primary hdd error) and i connected it to my other working computer to see if there was life in it. I was able to read files on the HDD and stuff but it turned out later that it was defect anyway and that I couldn't reinstall windows on it or install much or anything. Was impossible.

Anyway the day after i disconnected that HDD the computer froze on the desktop screen completely and i resetted computer only to find cmos checksum error and floppy disk error message (which would mean failure to read my old HDDs i guess rather than a floppy disk). Actually before that the monitor would often give me "no signal" error or something of that sort when i would start up, and the frequency of that happening has been getting higher and higher until now it won't even show anything, ever.

The times it did get signal there were those checksum error messages on startup. Pressing F1 to continue resulted in a blue screen and an auto restart. So i went into bios settings and changed things but to my surprise the settings did not save any more no matter what.

So did the hard disk somehow destroy my motherboard? And will it do the same to any other computer I connect it to? Fixed up the original pc by getting a completely new hard disk and installing everything from scratch, but I'm afraid of connecting this "broken" one to the computer to transfer my files in fear of it killing my system the same way.

could it be something completely different like power supply malfunction?

no its not the hdd thats causing the motherboard to fail. maybe the board was already bad. but run some tests on the hdd. it won't destroy anything else. the psu may be the problem, but im betting just the board. if the hdd passes the test, you know its the psu or mobo.

Checksum errors and not being able to save BIOS settings sounds like you need a new CMOS battery. Start by replacing the battery, reconfigure your BIOS and see if the other problems remain.