My iMac stopped reading my external SSD recently

ive had my iMac since last Christmas, and my dad gave me an external ssd to go with it. I've had it plugged in ever since and I use it to store my music project files and samples. a few days ago upon a restart, the external ssd did not appear on my desktop or finder. this scared me, I tried replugging it in and waiting but it wouldn't show up. eventually I connected it to my brothers windows pc to see if it still worked. his pc read my ssd and asked if we wanted to fix it. he clicks the fix button and I plug it back into my Mac and it reads it now. I have to do this every time I turn on my Mac because it stops reading it again after it's been turned off and on....anyone know why this is happening?