Have you checked out the system software version installed on your 550 ?
The system software was last updated on July 5th 2012 - that was version 5.00.
SDHC capability wasn't added until v3.00 - but I have no idea when that was.
https://www8.garmin.com/support/downloa ... sp?id=1671
Larger capacity SDHC cards didn't become available until relatively recently. I do not know how the addressing system works with SDHC cards - but this might give a clue:
https://en.wikipedia.org/wiki/SD_card#: ... patibility.
It talks about SDHC cards having a capacity up to 4GB
Later it talks about the newer SDHC cards which can access up to 2TB - but these are properly referred to as SDXC or SDUC.
It is possible - but I don't know for sure - that a larger capacity SDHC card is not being read properly by software that was designed before the later style of SDHC cards was introduced.
I remember at the time that many people were saying that they put larger capcaity cards into their Zumo and that it read them OK - but that may be the case for the first 4gb of storage, but then fail for the memory beyond that.
Something like - it can access memory locations that are numbered up to 4Gb, but like a car odometer, when it reaches the maximum number of digits, it goes round the clock to zero. So memory location 4Gb +1 is actually back to memory location number zero. The data will then make no sense and it will never find the end of a file that it is trying to read.
(ish)
I'd be tempted to check your software version on your Zumo 550 - make sure that it is version 5.00 and also try your Zumo with a lower capacity card, try that see if it works with smaller amount of data. Say initially with between 1 and 2Gb.
Then try it with between 2 and 4 Gb. Then see if it fails beyond 4Gb. I believe that SDHC cards have to be formatted as FAT32
OR
You could have a rip-off card - labelled as one size but actually containing much less available addressable storage. I doubt anyone would rip off a 4GB card nowadays with 256GB being readlity available. But if it is an old 4Gb card, it may be the case. 4Gb is a heck of a lot of storage space. Most people got nowhere near to storing that much data when they first came out !!
4Gb = storage for about 4.3billion characters. At 60 lines per page, 80 character wide, that is 0.9 million sides. If one side of paper was written in a diary for every day from the year 0, then there would still be enough space for the next 425 years.
Having said that, the amount of data in a modern routing map could gobble it up very easily.
