Mem cache is definitely still a thing. Non-volatile storage has gotten faster in recent years but it’s still not as fast as RAM. Depending how his system is configured, data loss is definitely possible.
That being said, unless it was one big file and he lost critical data that made the file readable (say an MBR on a disk image) there’s no way he should have lost 44GB.
If you’re going to directly connect them using just a single cable then it needs to be a crossover cable and he’ll need to set the IP manually on each end. Most people don’t have a crossover cable lying around. It would be easier to plug both computers into a router/switch and do a network transfer or just get a USB adapter for whatever kind of drive is in the old computer.
Edit: it’s been a while since I needed to dot do that and apparently modern Ethernet doesn’t need specialized cables anymore. TIL.
Oddly, Gentoo was where I started out when I got serious about using Linux. That was when I was in my 20’s and I wanted to get every last bit of performance out of my computer. Also, breaking stuff was fun and gave me a chance to figure new stuff out.
Now I just want stuff to work and be relatively up to date. So I use Debian testing.