New external hard drive not showing

If all else fails and you can return the Sabrent for a refund I suggest you get an external SSD from OWC (MacSales.com). Contact their Customer Support and find out which of their units will work with your iMac model. They have excellent support and products.

Going from a Thunderbolt Mac 1 to a Thunderbolt 3 drive is not possible. You need to either locate a Thunderbolt 1 drive (contact www.macsales.com ) or you need a new(er) iMac that supports Thunderbolt 3.

I would speak to Sabrent support to see if you can somehow do that. Let them know your computer is Thunderbolt 1 and ask if you can connect and use a Thunderbolt 3 drive.

Well I think that tells you that what I said about it not being reverse compatible may be correct.