

Are the read-write speeds sufficient for a server?
Depends.
For most of us, most use-cases, yes. I have 2 externals on USB that are no slower than my ancient NAS that only does 100Mbit Ethernet (honestly they’re probably faster).
Now if you’re running a transactional database, then externals are bad for throughput, but also stability/reliability and heat.
One thing I rarely see mentioned here is that external drives lack any cooling. This is fine for most uses-cases where people will intermittently copy data to them. But of you try sustained writes you can watch the temps climb in minutes. Which is why both of mine have old, large case fans on them (duct taped in place no less). They’re really quiet especially since I run them at 5v instead of 12V.
Externals are generally recommended against of it can be avoided. But of it’s what you have, run it, just know the lifespan is likely limited, and they’re not to be trusted from stability standpoint - always have redundancy/backups for important data.
My externals are part of my local redundancy - they replicate my main data drive, which is also replicated to my ancient NAS. So I have 3 local copies of everything.



Expand on your file sharing model/needs/usage pattern. .
Syncthing synchronizes specified folders. It’s quite configurable, but it’s really meant for a stable, regular syncing process. If you need more ad-hoc, it can do it but it’s not like using a network share.
Resilio Sync has a neat feature - selective sync, that allows you to grab specific files from a shared folder ad-hoc, rather than always sync all files all the time. I use it to grab media files (multi-gig) from my server when traveling.
IIRC, both have a “send only” setting for a sync job, so everyone could simply share to a specific folder, and have selective sync enabled for that folder… I think.
Both are Windows/Linux/iOS, and I think they’re both Mac.