You need to remember when you installed High Sierra on the SSD from a different system your 2009 iMac didn’t get its firmware updated as you hadn’t installed High Sierra on it directly.
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It sounds like your SATA to USB adapter is not able to support the Crucial SATA III (6.0 Gb/s) SSD.
Yep! Thats the expected reaction !!
You need to remember when you installed High Sierra on the SSD from a different system your 2009 iMac didn’t get its firmware updated as you hadn’t installed High Sierra on it directly.