Deleted files/folders after First Aid

I doubt first aid damaged anything. However, it's very possible the SD card was corrupted. How? Who knows; things happen. SD cards are not the best choice for long term storage of important documents; in my opinion they are the absolute last choice. I get they're convenient, small and all that but of the ones I've owned over the years, I now have two remaining I never use. If your card shows to be empty of data, it's likely just that ... empty of data.

A far better backup solution is external HDDs. The technology is mature and for the most part very reliable. Even then, I replace them at 5 year intervals because all devices are working toward their demise regardless of how expensive or highly rated they are. Time Machine (or perhaps Carbon Copy Cloner or SuperDuper!) is your friend. Every one of our 5 Macs have two external drives for backup redundancy as I've lost data too. In fact, there are two kinds of computer users: those who have lost data, and the who will. My solution is a solid back up plan, a discovery recovery plan and the discipline to keep them both working.

You can use it to store computer data of pretty much any type. Files are files but if you put non Apple format files on it, your Mac won't read them natively.

Don't make the mistake of using a data storage drive for Time Machine. You can do that but it's a decidedly bad mistake. TM should have a dedicated drive, better yet a couple of them for enhanced data security.

All drives of any time need to be formatted for the machine they are to be used on. Unless you create partitions, which is not always a good idea, the whole drive must be a single format. If two devices can use the same format, yes you can store both types on the same drive. But then if you lose the drive, your lose data from both machines. Drives fail; not partitions.

Gotta love spellcheck: "discovery" = "disaster"