@uski
Hopefully your problem isn't the same as the eTrex 20 device you mentioned, it seems it had an insurmountable problem flashing any new firmware version probably because someone had previously flashed a custom version fw.

Certainly some devices are incapable of flashing newer fw version if there are several versions skipped. It also seems that Garmin might have abandoned V2.10 probably due to it being 'buggy', they have a history of doing that and it's further evidenced by examining their changelog of SP i3's fw versions:
Code:
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The original RTM version was 2.0 but the next release shown is 2.40. As your device has V2.10 loaded clearly there were three 'duds' in between 2.00 and 2.40 which were subsequently withdrawn or some were not even publicly released.

Old devices connect to PC in 'Garmin Mode' which is very similar to the preboot mode of modern devices and both require Garmin USB Drivers to be loaded on the host PC so ensure that the SP is seen in Windows Device Manager under GARMIN Devices when connected by USB. If it's not then load the latest USB Drivers: [Only registered and activated users can see links. ]. Also be sure to only use a Desktop rear USB port directly, don't use a front port of the PC, any USB3 port or a hub. Windows 10/11 can certainly cope with Updater.exe V2.80 regardless that it's a very old application so there's no need to find an earlier OS.

When it's visible in Device Manager it can also be seen in [Only registered and activated users can see links. ] to check its HWID and you should do that because there's 2 possible versions, HWID 0429 and 0505, and we may need to flash it manually if running the V2.40 EXE won't work. Extract the linked ZIP file to find StreetPiloti3_240.exe:
Spoiler: Click for Link
[Only registered and activated users can see links. ]

Loading V2.40 successfully should then allow you to update to V3.40 directly however i also have all other versions from 2.50 to 3.20 should another incremental update be necessary.