Beddhist - gmapprom1.img is not for you only. It is from software source and must work on all version in Nuvi together with gmapsupp.img and gmapprom.img!
On SD only gmapsupp.img and gmapsup1.img will work from 4.70 version.
If it is not so you do something wrong. Or may be conflict with map and you need to try rename the extension (img to bak for example) of map one by one for finding this error. I often did it successful on new SW with support 2 map on SD. Good luck!
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