How do I upload photos from Samsung mobile to iMac?

Again, just making stabs in the dark. But when you are "uploading photos from samsung mobile to IMac," are you attempting to upload to Finder folders or to Photos. Going to Finder folders would have at least one less variable.

If it were an iPhone I would suggest a forced re-start. Have you tried that on your phone?

Cables can be a real problem-- have you tired swapping those out?

For the Mac side, to narrow things down, try these steps:

• Restart the computer (of course)
• Re-start in Safe Mode. This bypasses certain potentially disruptive processes, and it often helps. Safe Mode is different for different computers, so see this: 

           Start up your Mac in safe mode - Apple Support 

• Make a new user and see if the same problem recurs with it.  T

The idea is to find out if the problem is with your account, or with caches and login items, and stuff like that. One of these steps may fix the problem, but their failure to work also helps figure out what's happening.

Let us know what happens…

I have no experience with non-Apple phones, but with other non-Apple things what often works is to use the app Image Capture that's in your Applications folder.

Try that, and let us know how it goes…