So I was rushed (and being careless) and accidentally copied about 80 MB of a Raspberry PI image (SD card formatted extfs) over my external USB drive (4TB) that was formatted and encrypted (using Disk Utility but not sure of the specifics).
I tried following Recover deleted Encrypted OS X Extended volume and looked through Rebuild partition table on external HDD - not writable but still haven't managed to get the drive recognizable by my laptop (Mac OS 10.13.6 - where the drive had been mounted previously).
Running TestDisk did find these partitions (I assume from the backup GPT?) before I started making any changes:
#1557837238 Disk /dev/rdisk2 - 4000 GB / 3726 GiB - 7814037167 sectors 1 : start= 40, size= 409600, Id=00, P 2 : start= 409640, size=3518398048, Id=00, P 3 : start=3518807688, size= 262144, Id=00, P
(Note that I think the sizes may be incorrectly reported as I didn't change the disk geometry before running the scan)
Following @klanomath's excellent instructions in the first answer above, I did recreate the partition table, so now
gpt -r show /dev/disk2 does give:
start size index contents 0 1 PMBR 1 1 Pri GPT header 2 32 Pri GPT table 34 6 40 409600 1 GPT part - C12A7328-F81F-11D2-BA4B-00A0C93EC93B 409640 7813365343 2 GPT part - 53746F72-6167-11AA-AA11-00306543ECAC 7813774983 262144 3 GPT part - 426F6F74-0000-11AA-AA11-00306543ECAC 7814037127 7 7814037134 32 Sec GPT table 7814037166 1 Sec GPT header
diskutil cs list still reports
No CoreStorage logical volume groups found.
I'm unfortunately out of my element here and not sure how to proceed.