@GuMoM
Before you give up and buy another one, be aware that there's also a lesser-known 3rd US/EU Nav VI with 006-B4044-00, issued later than the other two (2584 and 2842) as indicated by its higher HWID numerals and with firmware Version 10.00.

The devices with HWID of 4044 have a different LCD screen than the earlier 2584 US/EU devices and were initially issued as replacements for the many 2584 devices which experienced screen fade/ghosting several years ago. You can read about a member who mistakenly flashed the wrong fw to his newer 4044 Nav VI here: [Only registered and activated users can see links. ]. In late 2022 at the time of that thread there was no fw available on the Garmin servers for 4044 devices, however since then Garmin has issued V10.10 on March 25, 2024:
Code:
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It seems likely that your device is HWID 4044 due to you reporting that Updater.exe will not load RGNs named 258401000590.rgn or 284201000280.rgn and gives you the "no updates available for your device" message. However, do not try to load ANY further RGN files until you are 100% certain of its HWID. You'll need to use G7ToWin to obtain it in preboot mode unless you have a previous backup of the device's GarminDevice.xlm file. Read here: [Only registered and activated users can see links. ].

Post again if you need more assistance.