The tax folders get backed up to a second hard drive on my machine, and then to Carbonite, with my other documents.
It's also backed up to a cloud server with borg, which is encrypted and doesn't trust the endpoint.
Aside from that, I keep all the paper copies of forms I receive in a filing cabinet. In January each year, I put the last year on a folder and just throw all my paperwork in there as it arrives. I have folders going back to 1999, the year I was married.
* First, I am about 99.9% paperless. Anything older than prior year is electronic. Some current year documentation is not yet scanned, but will be during the tax prep season
* Second, backups. Follow the rule of three. One offsite (crashplan), two onsite (main computer + storage device), three different file systems (whatever crashplan uses, OSX, Netgear / USB). I test my offsite recovery monthly with a sample selection.
* Three, I have a single precise location on computer with directories for each tax year.
* Four, I have subdirectories under each tax year that have a consistent retention requirement. 1 year after filing, 3 years, 5 years (FBAR), 7 years, until my retirement so +30 years, and until my death so +50 years, and active meaning do-not-delete until the tax event closes whereupon the retention period can be set. So like US bank statements that have a 1099, statements go into directory A with a 1 year retention. 1099s go into a different directory with a 3 year retention. A purchase of a new stock, the confirmation for that transaction would go under "active" meaning there is no deletion date, and that confirm would stay under "active" until the stock position was sold. At time of sale, that data is refiled into the correct tax year, where the appropriate retention period is applied to the info.
* Five, I use an application called TagSpaces, create a date specific retention framework, then tag each sub-directory from steps three and four with a destruction date. So tag "2019-12-31-7yr" was applied I think to tax year 2011. This past new year, I checked all objects tagged "2019-12-31-7yr", made sure nothing was misfiled under the tag, and then delete the object. Eventually a specific tax year will only have things intended to keep until retirement and until my death.
Tbh, i don't use it yet, but will once i get my homelab up and running (should be any moment cough)
I have a google drive folder with the information I need (bills, returns, forms, etc).
On top of that I have a google spreadsheet that links to the file and calculates amounts (like VAT paid) extracted (by hand) from the files. This acts both like an spreadsheet and an index to the data ;-).
Everything is shared with my SO just in case something happens to me.
It lives on an external drive that is backed up to Backblaze. Since it changes fairly infrequently, I'll also manually drop copies of it on other clouds occasionally.
At minimum three copies, one offsite in case of disaster and and preferably offline in case of ransomware. The details matter less unless you're worried about someone leaking your W-2 information?
I like to keep my personal life simple, so there is not much documents overall and i pretty much never needed access to anything older than weeks. When I was freelancing I had a more elaborate document management system in place regarding that work, but now this works just fine.