Hey there!
This is a long thread, so I haven’t read it all.
However, I’ve found that ANY time, I start to use an external hard drive with a Mac, I make SURE it’s formatted in a way that will always be able to read/write to/from. Although I definitely know software and hardware fails at times, which, to me I expect, despite which brand of computer you have, so routinely keep a few backups, as external hard drives can hole a lot of storage while remaining fairly affordable.
Formatting your external hard drive from the get go should, (but isn’t guaranteed to), keep your data safe. Which is why if I start backing up a new Mac, I check the formatting, storage, etc.
In each case, I start the formatting in Disk Utility, on the new OS’ make sure to select on the top menu: View > Show All Devices. Then select the TOP level of your external hard drive, then select the Erase tab:
When I plan to user a drive solely for Mac, I format as so:
Name:
—Give it a name.
Format:
—Mac OS Extended (Journaled), or APFS if necessary.
Partition Map:
—GUID.
If I plan on using the external hard drive for both Mac AND Windows OS’, erase the same way using:
Name:
—Give it a name.
Format:
—ExFAT.
Partition Map:
—Master Boot Record.
This should assure you that you won’t have problems reading from/writing to, these drives. Although formats change every now and then. And likewise external hard drives do fail or get corrupted and need to be erased. (As internal). Which is why it’s best practice to have a backup of a backup.
Hard drives do have a life span, so unfortunately they are not the fountain of life. However are readily available for decent prices, and do come in handy. Likewise cloud services are another option, (iCloud, Dropbox, One Drive, etc. etc.). However, even data stored in a cloud service should be backed up elsewhere, as servers do fail, no matter the company. Likewise, data gets corrupted, for a number of reasons, e.g. the account is constantly attempting to upload, or a device is attempting to download more data that it can handle or has room for. (One example).
Hope this helps someone, otherwise, best of luck, everyone here is here to help.