Did some tests with 4 LM devices. 2 recently updated with GE, 2 needing update. None of them fleet devices supporting FMI_HOS 1.0 [i don't have any of those in my stock presently]. It happily adds the multiple 'non-fleet' devices:

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But on starting to update, it does throw an error message but will finalise the update on the first device if the message is ignored by the user however won't move on the the next without restarting:

n0mv

When the program is closed and restarted, it does the same with the next device requiring update:

emf7

Info on all devices saved previously in GarminExpress is accessed by Garmin Fleet Updater, even if they are not connected:

tjn9

But it's a bit sneaky, the connected nuvis lose their drive letter and are only accessible via a folder 'nuvis' created in the root of the system drive, done in the background without reference or advice to you. In the 'nuvis' folder are next level folders with the UID of the individual unit, and in them a folder 'Device' via which the internal files system of the unit is accessible to browse:

bekf

If you delete a UID folder, you then can manually add a drive letter to access the files system normally. I'm sure a reboot would then add a letter automatically provided the folder has been deleted first. The folder has to be permanently deleted btw, if it's in the Recycle Bin it still shows as being accessed via there in Disk Management. A 4GB microSD in the 3790 could still be accessed normally as it didn't have it's drive letter removed. Not sure what would happen if it was needed for a map update because of insufficient space on the unit though:

g4au

Interesting program though and probably a Godsend for busy time-poor and lowly IT-skilled fleet managers, or as Giomen said 'secret weapons for corporate clients' lol. It seems a bit further developed and more stable that GE was on it's release which was only deep Alpha at best i reckon.