You raise some good questions regarding fault-tolerance/redundancy of the hardware/platform etc.
From what I understand, this is a working BETA and as such to be used with caution.
My second point which is to do with transactions is that its really quite simple, if you are carrying out transactions then they are always going to be available providing you make a record of your priv-key and make the backup of your LN channels.
This is one of the beauties of Bitcoin a copy of the ledger is constantly being duplicated and shared around and validated by the nodes /miners on the network. This is the blockchain.
Even in a catastrophic event and you lose your data there is usually a way to gain access to said funds
which you could then restore back in after you format the device again…
There will always be edge case scenarios or scenarios which will almost certainly break the system.
But my main point is a node is not supposed to be powered on or off frequently. They are supposed to be Servers and as you know servers are up 24/7 365+ uptime…
In the case of sudden power loss this is covered in the intro docs I read right here on umbel community forum.
It states something like “If possible make sure that your Umbrel node is powered through a UPS device to cover you in the extreme circumstance of power loss…”
As for the OS umbrel is built around, its *nix and derivative of Debian which is rock solid. It has been powering web front/backends for ages. on top of this the way the umbrel devs have developed this OS / Platform is very good… Everything is compartmentalised/abstracted away from underlaying layers and running in their own container. This in itself provides a degree of redundancy…
Directly to your questions you can query what the filesystem is take a look at some basic linux commands:
sudo fdisk -l
You will notice dev/sda is GPD
/dev/mmcblk0p1 W95 FAT32 (LBA)
You will need to DYOR on the filesystem types and decide if this is what you are willing to live with…
I for one am very impressed with the project and the product to date.
I think the memory card is used as a swap and as the installer.
Hope this helps !