This info you've heard is wrong: 'I thought that if I removed the gmapprom.img the nuvi wouldn't boot'. Not so, the map img file naming in not important in the boot process itself and the unit will then read it if the path is correct and the naming convention is correct, 'anyname'.img in Map folder on unit or card, gmapprom.img or gmapsupp.img etc in unit .System folder. The important thing is that the detail map is present and readable.

What you may have heard is that occasionally a unit will report 'no detail maps present' after booting even though there is a detail map on the media card correctly but none on the unit. It can be fixed by putting the smallest detail map you can find on the unit [I use Raratonga if i get that rare problem] and all others on the card. I have a 260W which can't fit a World basemap and all of AU NZ and will only acknowledge it on the SD if some tiny detail map is on the unit.

You problem is almost certainly one of conflict between the maps for some reason. Either entirely remove the UK/IR map or rename it as x-trail suggested then use the full EUU, or put only rest of EU less UK/IR from MapInstall, on the microSD. Do a hard reset/clear NV if still having problems, its the best first aid for any stroppy unit.

I guess you've done a typo with the name of the JCV file in the initial post [D3048120A0]. Correct is D3048120A. You could try moving that back to the JCV folder in .System although your reported path is correct for the microSD. Reset might help that too. Also try another card, garmins are flaky with some 'no-brand' cards.

EDIT: Just noticed this when i merged your 2 posts:
Quote Originally Posted by mallard2 View Post
......... I have the full euro map on the SD as gmappsupp.img .......
Try renaming that to 'gmapsupp.img' first if it's in the card's Garmin folder.