See if there is some known problem with the SSD and a firmware fix. I know my Crucial 512GB SSD that I now use in my MacBook had an issue where it would randomly disconnect and seem to be dead. IIRC there was a firmware fix for the issue.
You might see if something similar is happening with your Samsung SSD.
		
		
	 
The installed firmware version was RVT01B6Q. I am now trying to install version RVT04B6Q via a USB bootable drive. I'll let you know if it worked.
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			The Samsung 860 Evo is a quality SSD.
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I think this points to a SATA cable failure. Did this behaviour happen directly after you upgraded to the SSD?
Could you install a SMART data reading tool, such as Drive DX (this has a free 15 day trial period) and check for the 'UDMA CRC error' parameter? Counts for this parameter means that data got corrupted between the drive and the motherboard, as it would be typical for a failed cable (for instance). The counts also will go up quickly.
Also, it might be useful for the ones who can dive deeper to post the ETRE check report here.
Best,
Magnus
		
		
	 
Dear Magnus,
Thank you for your reply. I downloaded Drive DX and indeed it shows an error in UDMA count (see attached screenshots). Many thanks for your suggestion. I will call somebody who will fix the cable for me. 
You can find the complete ETRE report below:
 
EtreCheck version: 5.5.1 (5096)
Report generated: 2020-03-08 08:59:41
Download EtreCheck from https://etrecheck.com
Runtime: 2:41
Performance: Excellent
Sandbox: Enabled
Full drive access: Disabled
Problem: Other problem
Description: 
Prohibitory
Major Issues: None
Minor Issues:
    These issues do not need immediate attention but they may indicate future problems or opportunities for improvement. 
    
Configuration profiles present - This machine has configuration profiles. These are sometimes used by adware and malware.
    
High battery cycle count - Your battery may be losing capacity.
    
Limited drive access - More information may be available with Full Drive Access.
Hardware Information:
    MacBook Pro (13-inch, Mid 2012)
    MacBook Pro Model: MacBookPro9,2
    1 2,5 GHz Dual-Core Intel Core i5 (i5-3210M) CPU: 2-core
    8 GB RAM - At maximum
        BANK 0/DIMM0 - 4 GB DDR3 1600 
        BANK 1/DIMM0 - 4 GB DDR3 1600 
    Battery: Health = Normal - Cycle count = 1599
Video Information:
    Intel HD Graphics 4000 - VRAM: 1536 MB
        Color LCD 1280 x 800
Drives:
    disk0 - Samsung SSD 860 EVO 250GB 250.06 GB (Solid State - TRIM: Yes)
    Internal SATA 6 Gigabit Serial ATA
        disk0s1 - EFI (MS-DOS FAT32) [EFI] 210 MB
        disk0s2 [APFS Container] 249.85 GB
            disk1 [APFS Virtual drive] 249.85 GB (Shared by 5 volumes)
                disk1s1 - S*******************B (APFS) (Shared - 10.83 GB used)
                disk1s2 - Preboot (APFS) [APFS Preboot] (Shared)
                disk1s3 - Recovery (APFS) [Recovery] (Shared - 524 MB used)
                disk1s4 - VM (APFS) [APFS VM] (Shared - 4.30 GB used)
                disk1s5 - S********************* Data (APFS) [APFS Virtual drive] (Shared - 90.38 GB used)
    disk2 - WD Elements 1078 1.00 TB
    External USB 5 Gbit/s USB
        disk2s1 - EFI (MS-DOS FAT32) [EFI] 210 MB
        disk2s2 [Core Storage Container] 999.83 GB
            disk3 - E*****m (Mac OS Extended (Case-sensitive, Journaled, Encrypted)) 999.46 GB (922.91 GB used)
        disk2s3 - Boot OS X (Journaled HFS+) 134 MB
Mounted Volumes:
    disk1s1 - S*******************B
        249.85 GB (Shared - 10.83 GB used, 144.97 GB available, 143.66 GB free)
        APFS
        Mount point: /
        Encrypted
        Read-only: Yes
    disk1s3 - Recovery [Recovery]
        249.85 GB (Shared - 524 MB used, 143.66 GB free)
        APFS
        Mount point: /Volumes/Recovery
    disk1s4 - VM [APFS VM]
        249.85 GB (Shared - 4.30 GB used, 143.66 GB free)
        APFS
        Mount point: /private/var/vm
    disk1s5 - S********************* Data [APFS Virtual drive]
        249.85 GB (Shared - 90.38 GB used, 144.97 GB available, 143.66 GB free)
        APFS
        Mount point: /System/Volumes/Data
        Encrypted
    disk3 - E*****m
        999.46 GB (922.91 GB used, 76.55 GB free)
        Mac OS Extended (Case-sensitive, Journaled, Encrypted)
        Mount point: /Volumes/E*****m
        Encrypted
Network:
    Interface en0: Ethernet
    Interface en4: iPhone
    Interface fw0: FireWire
    Interface en1: Wi-Fi
        802.11 a/b/g/n
    Interface en3: Bluetooth PAN
    Interface bridge0: Thunderbolt Bridge
System Software:
    macOS Catalina 10.15.2 (19C57) 
    Time since boot: About 10 hours
Configuration Profiles:
    This computer has configuration profiles installed.
Notifications:
    Notifications not available without Full Drive Access.
Security:
    Gatekeeper: Enabled
    System Integrity Protection: Enabled
    Antivirus software: Apple
System Launch Agents:
    [Not Loaded] 17 Apple tasks
    [Loaded] 170 Apple tasks
    [Running] 124 Apple tasks
System Launch Daemons:
    [Not Loaded] 35 Apple tasks
    [Loaded] 180 Apple tasks
    [Running] 120 Apple tasks
    [Other] One Apple task
Launch Daemons:
    [Loaded] com.apple.installer.osmessagetracing.plist (Apple - installed 2019-09-19)
    [Loaded] com.microsoft.autoupdate.helper.plist (Microsoft Corporation - installed 2020-03-06)
    [Loaded] com.microsoft.office.licensingV2.helper.plist (Microsoft Corporation - installed 2019-11-11)
    [Loaded] com.skype.skypeinstaller.plist (Skype Communications S.a.r.l - installed 2017-11-29)
User Login Items:
    [Not Loaded] ShazamHelper (App Store - installed 2020-03-07)
        Modern Login Item
        /Applications/Shazam.app/Contents/Library/LoginItems/ShazamHelper.app
    [Not Loaded] StartUpHelper (Spotify - installed 2019-12-18)
        Modern Login Item
        /Applications/Spotify.app/Contents/Library/LoginItems/StartUpHelper.app
    [Not Loaded] HelperApp (App Store - installed 2019-09-18)
        Modern Login Item
        /Applications/WeTransfer.app/Contents/Library/LoginItems/HelperApp.app
    [Not Loaded] WhatsApp Login Helper (App Store - installed 2020-03-07)
        Modern Login Item
        /Applications/WhatsApp.app/Contents/Library/LoginItems/WhatsApp Login Helper.app
User Internet Plug-ins:
    User Internet Plug-ins need Full Drive Access
User Audio Plug-ins:
    User Audio Plug-ins need Full Drive Access
User iTunes Plug-ins:
    User iTunes Plug-ins need Full Drive Access
Safari Extensions:
    AdBlock Engine (App Store - installed 2019-11-06)
    AdBlock Icon (App Store - installed 2019-11-06)
    LibraryAccessSafari (App Store - installed 2020-03-07)
Time Machine:
    Time Machine information not available without Full Drive Access.
    One local snapshot
    Oldest local snapshot: 2020-03-07 22:37:04
    Last local snapshot: 2020-03-07 22:37:04
Performance:
    System Load: 3.03 (1 min ago) 2.86 (5 min ago) 2.64 (15 min ago)
    Nominal I/O speed: 3.25 MB/s
    File system: 37.34 seconds
    Write speed: 492 MB/s
    Read speed: 397 MB/s
CPU Usage Snapshot:
    Type Overall
    System: 9 %
    User: 16 %
    Idle: 74 %
Top Processes Snapshot by CPU:
    Process (count) CPU (
Source - 
Location)
    Other processes 77.16 % (?)
    trustd 16.44 % (Apple)
    EtreCheck 7.87 % (App Store)
    CoreServicesUIAgent 0.96 % (Apple)
    CalendarAgent 0.03 % (Apple)
Top Processes Snapshot by Memory:
    Process (count) RAM usage (
Source - 
Location)
    EtreCheck 522 MB (App Store)
    suggestd 90 MB (Apple)
    accountsd 80 MB (Apple)
    CalendarAgent 76 MB (Apple)
    Spotlight 71 MB (Apple)
Top Processes Snapshot by Network Use:
    Process Input / 
Output (
Source - 
Location)
    Other processes 889 KB / 162 KB (?)
    CalendarAgent 130 KB / 37 KB (Apple)
    SystemUIServer 0 B / 192 B (Apple)
    mdworker_shared 0 B / 0 B (Apple)
    spindump_agent 0 B / 0 B (Apple)
Virtual Memory Information:
    Physical RAM: 8 GB
    Free RAM: 1.73 GB
    Used RAM: 3.68 GB
    Cached files: 2.59 GB
    Available RAM: 4.32 GB
    Swap Used: 0 B
Software Installs (past 30 days):
    Install Date Name (
Version)
    2020-03-06 Microsoft AutoUpdate
    2020-03-06 MRTConfigData (1.56)
    2020-03-06 XProtectPlistConfigData (2115)
    2020-03-06 Microsoft Word
    2020-03-06 Microsoft Outlook
    2020-03-06 Microsoft PowerPoint
    2020-03-06 Microsoft Excel
    2020-03-07 EtreCheck (5.5.1)
    2020-03-07 WhatsApp (0.4.930)
    2020-03-07 Library Access (2020.3)
    2020-03-07 Shazam (2.10.0)
Diagnostics Information (past 7-30 days):
    Directory /Library/Logs/DiagnosticReports is not accessible.
    Enable Full Drive Access to see more information.
End of report