Exact scenario here too, running Catalina on an External SSD. Same error number too. External drive worked fine running Catalina for over a year using the motherboard of a late 2015 MacBookPro running Sierra (the "32 bit software issue"). Battery on laptop was changed in mid-December. When I tried to connect with External Lacie 1Tb upon laptop's return, external drive would not mount. Disk Utility repair and mount failed, running Catalina on yet another 5Tb external drive and Sierra OS on laptop. 1Tb External drive was running Logic ProX and Final Cut Pro. Raw files backed up, but changes currently unattainable. Feels like some sort of permissions denial? Dunno. Does Lacie place empty space cushions in their drives, so they don't fill to the brim? I wonder if I remove/delete contents in VM or Recovery partitions (related to Catalina OS install?), if it will free up space for drive to mount? Wondering why there were no "disk full warnings." I thought there was 8 Gb of space left when I took in the laptop to Apple for battery repair...
ehMTee
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