Chosen Solution

First post here — hope someone can help. I restored onto a 2TB Crucial SSD (CT2000MX500SSD1) from Time Machine and attempted to replace the existing 500 GB HDD with it, but on booting it up I get an alternating question mark folder icon with the Apple logo and a cross icon. I can successfully boot up the iMac with this new SSD via an external caddy (through the USB port) — but not internally. I double-checked the internal connections to the SSD with the SATA data and SATA power cables but they seem nicely connected and I’m still getting the same issue. I then put the original 500GB HDD back to check I hadn’t damaged the cables / sockets but it booted up fine. I then replaced it back with the new SSD and still got the non-detection issue. On a side note, not sure if this is related, but might be worth mentioning. When I tried to boot up externally with the original HDD via the caddy, the iMac wouldn’t detect it, but does detect the old HDD internally — and as mentioned, the caddy works fine booting up the iMac with the SSD. Thanks for any help. Update (03/20/2021) When I boot up with the Option key, it shows me the disk (SSD) and when I select it I get the flashing question mark folder > Apple icon > circle with line through icon — all on a rotation These are the steps I took:

  1. I removed the original 500GB Seagate HDD from my iMac and attempted to fit the SSD directly into the space (straight from the packaging without doing any preparative work) utilizing a new SATA cable and 2.5" to 3.5" mounting kit
  2. I then rebooted and hoped to see the new SSD to perform a restore (tried holding down the Option key (for the startup manager) and again with Command+R (for recovery)) but couldn’t see it
  3. I opened the iMac again and double-checked the internal connections to the SSD with the SATA thermal sensor, data and power cables but they were nicely connected and I rebooted but still got the same issue
  4. I took the SSD out of the iMac and used my 2018 MacBook Pro to erase and format it with the Mac OS Extended (Journaled) option
  5. I then tried refitting it again in the iMac and rebooting. Still no joy
  6. I tried again without going via the thermal sensor SATA cable but instead going directly with the iMac SATA cables — still no use.
  7. I then decided to use an external caddy for the SSD to connect to the iMac via USB where I could successfully initiate an internet recovery to install MacOS and then do a TimeMachine restore
  8. Whilst it was connected externally via the caddy it booted up okay and I could use the iMac as normal — but obviously I want to install it internally
  9. I then opened up the iMac again and tried to install the SSD, rebooted and it failed again
  10. I then put the original 500GB HDD back in to check I hadn’t accidentally damaged the cables / sockets but it booted up fine. I then swapped it back with the new SSD but again got the non-detection issue (flashing question mark folder > Apple icon > circle with line through icon — on a rotation)

Your system is messed up on which drive to boot from! You’ll need to go into your systems preferences to reset it. But you first need to boot up with your SSD mounted internally. To do that you need to use the startup key Option (⌥) key to enable the Startup Manager so you can select the SSD. Once your system is booted up go into the preference setting > Startup Disk to set it permanently. Reference: Mac startup key combinations