... but it works fine: it transfers files on Garmin Connect, as it always did. Not even Mac (macOS High Sierra 10.13.4) recognizes Garmin 520 as a storage volume.
What cable are you using? The one provided with the 520? Have you tried a different cable?
Are you using a USB hub?
I've found that some cables are designed to only provide power. Others are marginal for data transfer (especially when they are longer in length). Also, some hubs don't work for some reason.
My recommendation is to try a cable that you know works for data transfer with other devices.
I read other posts, and if I have understood well, the problem is incompatibility between GE 6.3.0 and macOS High Sierra 10.13.4. The question is: does Garmin intend to propose an update to solve the problem?
I add a detail. I removed 520 from GE, and now 520 is not present in GE. But if I connect 520 to Mac by USB, I can download some data (p.e. cycling routes) from my page on Garmin Connect to 520...
