You must tell me what the Delta Quadrant is like.. if I remember my Star Trek Lore correctly.
Watch out for black holes.
what would you buy to use as an automated time machine compatible back up drive that would be unplugged from two laptops. So basically both laptops would back up over Wi-Fi like I’ve always had it. 3-4TB would be plenty of space. Thanks
This is the Goldilocks Paradox. You want just right.. not too hot.. not too cold.
If money is no object.. go hot.. Synology NAS. Plug it into router.. there you have it. Wifi or ethernet connect via your router to the NAS. It has several major advantages.. It is FAST.. much faster than anything USB hanging off a router. It is safe and reliable.
It can do loads of stuff on your network.. e.g.. handle security cams, video store, itunes server, photos store. file sharing.
Everyone would be using them if they didn't cost so much.
A very popular DIYer form of this is an HP microserver running Xpenology which is free version of Synology OS. Actually any old PC can do this job. This is a medium variation.. cold on money.. hot on time.
If money is seriously short.. Go too cold.. And this is with strong DIY skills.. I would buy a Gen4 A1409 Time Capsule from eBay. It must be in working condition but they sell here for $50.. I have bought and sold lots for people. Does not matter if it is 2TB or 3TB although 3TB is preferred if it is cheap enough.. don't pay a lot extra. They have WD Green that are now at least 6-8 years old. Even an earlier one the Gen3 A1355 which mostly used a 1TB drive. And it could sell here for $20-40 are basically the same with poorer wireless which you are going to turn off. Don't buy earlier ones.. they are crappy with poor power supplies.
I would replace the disk with 4TB if required.. immediately in the case of 1TB samsung.. but if 2TB is enough.. why not use the drive from your Gen5. Before you put the base back on drill some holes in it.. and be prepared for power supply failure. I just use external supply. Don't put the rubber mat back.. use some feet. Lots of info on my website if you go back to the main pages. These earlier ones are just as fast on disk access as the Gen5, much better built, and easy to modify. Plug it into the main router by ethernet and turn off wireless. Throw the Gen5 in the bin. The problem is long term.. this is going to keep you going a few years with luck but eventually it is going to die. Apple intend everything to be cloud based by then I think. shudder!!
The middle of the road method. Neither too hot or too cold.
Do not buy the ISP router. Buy Asus or Synology. If you need a modem buy a pure modem with no router.. cheap and easy.
Plug the 3-4TB USB drive into it and setup Time Machine according to the relevant manual or online instructions.
If you have issues.. be ready to dump Time Machine for a decent alternative like Carbon Copy Cloner. License covers all computers in your home. It works hugely better than TM for reliability. You do need to put it into your costings. Actually on any Mac OS since Sierra I would say the same to any network target.