EPO

My limited experience is that EPO does get updated by the Explore app, but not very reliably. Right now, EPO on my device (in RemoteSW folder) is two days old and I've only synchronized via Explore for the past week. So somehow it works. But as mentioned before, deleting the file manually won't deliver it on the next sync. It gets only updated on its own weird schedule and I am yet to find any rhyme or reason when it decides to do so. I wouldn't really care if it's often enough so the file at least doesn't expire, but it seems that's not always a case.

On the other hand, yes, Garmin Express always updates the file, whenever you synchronize via computer. But that's hardly a solution, since I rarely even use a Windows computer these days and it really shouldn't be needed. One of the motivations for me to upgrade to M2 was that it can do firmware updates without needing to plug it in. My Fenix watch works fine without needing to be plugged in (except for map updates, which makes sense given the size), so hopefully M2 gets there too soon. 

The unit will acquire even with an expired EPO. It may take a bit longer. In theory, I would guess that the device would eventually acquire an updated almanac on its own. But it's hard to do this when the unit is moving. It takes 12.5 minutes to load the entire almanac. And an interruption in signal slows it down some more.

But isn't EPO better than almanac? From what I remember, almanac doesn't have enough precision to get position fix, it just tells the device what satellites to look for first and it seems to be less needed these days. The device still needs to get epehemeris data from the each individual satellite, to be able to use it to get a fix, and those data are valid for some 6 hours at best. EPO on the other hand has more detailed ephemeris data for all the satellites that's valid for maybe a week. But those are never broadcast by satellites themselves, because it's 1980s technology with very limited bandwidth. ;-)

But yes, EPO isn't that important, and with device like inReach, you still have to remember to synchronize it before the trip, which I normally don't do. If I just start tracking after having it off for two weeks, it will be expired no matter what Garmin does. M1 as far as I can tell doesn't use EPO and displays typical behavior of GPS receivers that don't - it takes 30+ seconds to get GPS fix in the best of circumstances and considerably longer if you're moving under tree cover (unless the device was on relatively recently). M2 on the other hand often rivals my phone in the speed of initial acquisition, and I suspect EPO has lot to do with that. But obviously, the whole GPS chipset is probably better now. 

I got this information from Wikipedia https://en.wikipedia.org/wiki/GPS_signals

A GPS message block contains among other things:

• Ephemeris: precise orbital information for the transmitting satellite. An ephemeris is valid for only four hours.
• Almanac component: status and low-resolution orbital information for every satellite. An almanac is valid with little dilution of precision for up to two weeks.

As mentioned by @twolpert, it takes a single satellite 12.5 minutes to transmit the whole almanac. Since the receiver is connected to multiple satellites, I assume it gathers the whole almanac faster.

The almanac serves several purposes. The first is to assist in the acquisition of satellites at power-up by allowing the receiver to generate a list of visible satellites based on stored position and time, while an ephemeris from each satellite is needed to compute position fixes using that satellite.

Garmin uses the term Extended Prediction Orbit (EPO), which I assume means the almanac. Further, I guess Glonass and Galileo using their own almanacs and perhaps Garmin gathers them all together in a single EPO file (just guessing).

schneeleopard said:

Garmin uses the term Extended Prediction Orbit (EPO), which I assume means the almanac

A logical conclusion.

However, while many Single-Band Garmin receivers use an EPO file, their Multi-Band devices use a very similar CPE file.

EPO = Extended Predictive Orbit

CPE = Connected Predictive Ephemeris 

As it appears Garmin Multi-Band devices are using 'ephemeris', I suspect the 'Extended' in EPO may indicate something similar.

I would caution anyone against making the common false assumption that EPO and CPE files somehow equate to receiver accuracy and/or precision, which they do not!

The EPO and CPE files are strictly used to improve time to first fix after device power up. When the EPO/CPE file is current, this time is nearly instantaneous and imperceptible to the user. When these files are missing or expired, the GPSr can require a minute, or several minutes to find it's location, just like in the 'good ol' days'.