Look for a Class 10 minimum. They're pretty slow compared to a SATA III SSD but should be more than fast enough for iTunes. A 256GB Class 10 costs around $40 or so.I don't know that it matters all that much. I have a Sandisk Extreme SD card in my iMac that has my large music library on it to avoid wasting the fast internal SSD disk space on iTunes music files.
I haven't done this with either my iMac at work or my Dell XPS desktop at home. External HDDs and SSDs are no problem managing, physically, when using a desktop computer. I have, though, done this with two Windows tablets, my old Dell Venue 8 Pro and my current Surface Go.Would like to use SD card slot on 2019 iMac for additional file/photo storage. Would generally leave the card in place in the slot. Is there a specific card that others have found reliable and successful for this use? Thanks!
I wouldn't do this.
1. SD cards are generally MUCH SLOWER than USB3 drives
2. It probably wouldn't be as "reliable" as an external drive. Too easily "dislodged".
Just get an external USB3 drive (platter based or SSD).
It will do the job better and faster.
As I pointed out, a Class 10 is 1/5 the speed of a SATA III SSD — doesn't matter how it's hosted on a 2012 or later Mac. That's slower than a 5400 rpm HDD. If Class 4 or 6, that's even slower.
If fast enough, then great, I suppose. Only the small ones are inexpensive. Above 128GB, they become expensive for what you get.