Hard to tell, but my guess would be a power issue.
Is the drive directly attached to the host? or is there a USB hub in the middle?
If Sandisk provided the USB-A<->USB-C adapter, then they're presumably saying running off USB-A is OK, but USB-C devices can pull (or, at least, ask for) more power than the old USB-A ports can provide.
Indeed, according to the spec sheet, the Late 2015 iMac sported USB 3.0 ports with a theoretical power limit of 5V/900mA, or 4.5W.
I don't know specifically which Sandisk drive you're using, but the one I checked supported USB 3.2. While it's backwards compatible from a data transfer standpoint, it's not clear what the drive's power requirements are.
You might glean more info from System Information.app with the drive plugged in.
Check the USB section in System Information and find your drive. There should be a line for 'Current Available' and 'Current Required'. Make sure there's at least as much current available as the device requires.
Also bear in mind that, depending on your configuration, the USB power may be shared across multiple devices, especially if you're using an unpowered hub. If that's the case, adding a powered USB hub, or plugging the drive directly into the iMac may help.
Of course, it may not be power-related at all, but that's my first thoughts.
